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ta Washington, D.C. : tb Office of Scientific Research and 
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1946. 

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ta Summary technical report of Division 16, NDRC ; tv v. 4 

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ta Includes bibliographical references (pages 137-140). 

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ta Half-title page: Summary technical report of the National Defense 
Research Committee. 

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ta "Manuscript and illustrations for this volume were prepared for 
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of Scientific Research and Development. This volume was printed and 
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ta LC Science, Business & Technology copy no. 5. t5 DLC 

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ta Infrared imaging. 

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ta United States, tb Cffice of Scientific Research and Development, 
tb National Defense Research Committee. 


Bibliographic Record #19037814 


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SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


This document contains information affecting the national defense of the 
United States within the meaning of the Espionage Act, 50 U.S.C., 31 and 32, 
as amended. Its transmission or the revelation of its contents in any manner 
to an unauthorized person is prohibited by law. 


This volume is classified RESTRICTED in accordance with security regu- 
lations of the War and Navy Departments because certain chapters contain 
material which was RESTRICTED at the date of printing. Other chapters 
may have had a lower classification or none. The reader is advised to consult 
the War and Navy agencies listed on the reverse of this page for the current 
classification of any material. 





Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia 
University Division of War Research under contract OEMsr-1131 
with the Office of Scientific Research and Development. This vol- 
ume was printed and bound by the Columbia University Press. 

Distributions of the Summary Technical Report of NDRC has 
been made by the War and Navy Departments. Inquiries concern- 
ing the availability and distribution of the Summary Technical 
Report volumes and microfilmed and other reference material 
should be addressed to the War Department Library, Room lA-522, 
The Pentagon, Washington 25, D. C., or to the Office of Naval 
Research, NaA^y Department, Attention : Reports and Documents 
Section, Washington 25, D. C. 

Copy No. 

5 


This volume, like the seventy others of the Summary Technical 
Report of NDRC, has been Avritten, edited, and printed under 
great pressure. Inevitably there are errors AAffiich have slipped past 
Division readers and proofreaders. There may be errors of fact not 
knoAvn at time of printing. The author has not been able to follow 
through his Avriting to the final page proof. 

Please report errors to: 

JOINT EESEAUCH AND DEVELOPMENT BOAPD 
PPtOGEAMS DIVISION (STPt EEEATA) 

AVASHINGTON 25, D. C. 


A master errata sheet Avill be compiled from these reports and sent 
to recipients of the volume. Your help will make this book more 
useful to other readers and Avill be of great value in preparing any 
revisions. 



SUMMARY TECHNICAL REPORT OE DIVISION 16, NDRC 

VOLUAIE 4 


IMAGE FORMING INFRARED 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 16 

GEORGE R. HARRISON, CHIEF 



VVASHSjii'iJ i GM* Om 





NATIONAL DEFENSE RESEARCH COMMITTEE 


35 -JAN12 

l W Lffrr. ^ J 


James B. Coiiant, CJiairnian 
Bicliard C. Tolman, Vice Cltcvirvian 
I\oger Adams Army Eepresentative^ 

Frank B. JeM'ett Xavy Representative^ 

Karl T. Compton Commissioner of Patents^ 

Irvin SteAvart, E.vecidive Secretanj 


^Army representatives in order of service: 

Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. Williams Brig. Gen. E. A. Regnier 

Brig. Gen. W. A. Wood, Jr. Col. M. M. Irvine 

Col. E. A. Routheau 


‘^Navy representatives in order of sendee: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Purer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
^Commissioners of Patents in order of service: 

Conway P. Coe Casper W. Ooms 


NOTES ON THE OEGANIZATWN OF NDEC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech- 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research jirojects on requests 
from the Army or the Navy, or on requests from an allied 
government transmitted through the Liaison Office of OSRD, 
or on its own considered initiative as a result of the experience 
of its members. Proposals prepared by the Division, Panel, 
or Committee for research contracts for performance of the 
work involved in such projects were first reviewed by NDRC, 
and if approved, recommended to the Director of OSRD. 
Upon approval of a proposal by the Director, a contract 
permitting maximum flexibility of scientific effort was ar- 
ranged. The business aspects of the contract, including such 
matters as materials, clearances, vouchers, j)atents, priorities, 
legal matters, and administration of patent matters were 
handled by the Executive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were; 

Division A — Armor and Ordnance 

Division B — Bombs, Fuels, Gases, & Chemical Problems 
Division C — Communication and Transportation 
Division D — Detection, Controls, and Instruments 
Division E — Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the j^articular field. The NDRC members then be- 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Exjfiosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 



NDRC FOREWORD 


A s EVENTS of the years preceding 1940 revealed more 
- and more clearly the seriousness of the world 
situation, many scientists in this country came to 
realize the need of organizing scientific research for 
service in a national emergency. Eecommendations 
which they made to the White House were given care- 
ful and sympathetic attention, and as a result the 
National Defense Eesearch Committee [NDEC] was 
formed by Executive Order of the President in the 
summer of 1940. The members of NDEC, appointed 
by the President, were instructed to supplement the 
work of the Army and the Navy in the development 
of the instrumentalities of war. A year later, upon the 
establishment of the Office of Scientific Eesearch and 
Development [OSED],NDEC became one of its units. 

The Summary Technical Eeport of NDEC is a 
conscientious effort on the part of NDEC to summa- 
rize and evaluate its work and to present it in a useful 
and permanent form. It comprises some seventy vol- 
umes broken into groups corresponding to the NDEC 
Divisions, Panels, and Committees. 

The Summary Technical Eeport of each Division, 
Panel, or Committee is an integral survey of the work 
of that group. The first volume of each group’s report 
contains a summary of the report, stating the problems 
presented and the philosophy of attacking them, and 
summarizing the results of the research, development, 
and training activities undertaken. Some volumes may 
be ^‘'state of the art” treatises covering subjects to 
which various research groups have contributed in- 
formation. Others may contain descriptions of devices 
developed in the laboratories. A master index of all 
these divisional, panel, and committee reports which 
together constitute the Summary Technical Eeport 
of NDEC is contained in a separate volume, which 
also includes the index of a microfilm record of per- 
tinent technical laboratory reports and reference 
material. 

Some of the NDEC-sponsored researches which had 
been declassified by the end of 1945 were of sufficient 
popular interest that it was found desirable to report 
them in the form of monographs, such as the series 
on radar by Division 14 and the monograph on sam- 
pling inspection by the Applied Mathematics Panel. 



Since the material treated in them is not duplicated 
in the Summary Technical Eeport of NDEC, the 
monographs are an important part of the story of 
these aspects of NDEC research. 

In contrast to the information on radar, which is 
of widespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of 
Division 6 is found almost entirely in its Summary 
Technical Eeport, which runs to over twenty volumes. 
The extent of the work of a division cannot therefore 
be judged solely by the number of volumes devoted 
to it in the Summary Technical Eeport of NDEC : 
account must be taken of the monographs and avail- 
able reports published elsewhere. 

Division 16 carried out a broad program in the 
fields of light and optics. Among the studies under- 
taken were a number involving the principles and 
techniques of camouflage, and perhaps the outstand- 
ing success achieved in this field was the development 
of the “black widow” finish for night-flying aircraft. 
Significant improvements were made in aerial map- 
ping and photography. Devices depending on the use 
of infrared light were developed for the detection of 
enemy craft, the recognition of friendly ones, and 
for intercommunication by voice and code. The 
sniperscope, using image-forming infrared rays, was 
a spectacular weapon which enabled our troops to fire 
accurately on an enemy 100 yards away in utter 
darkness. 

The Division 16 Summary Technical Eeport, pre- 
pared under the direction of the Division Chief, 
George E. Harrison, describes the technical achieve- 
ments of the Division personnel and its contractors, 
and is a record of their skill, integrity, and loyal co- 
operation. To all of them, we extend our grateful 
praise. 


Vannevar Bush, Director 
Office of Scientific Research and Development 



J. B. CoNANT, Chairman 

Research Committee 


V 













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FOREWORD 


AT THE TIME of its formation late in 1942, Divi- 
-Ol sion 16, the Optics Division of NDRC, was 
assigned both the general task of stimulating and 
supervising OSRD research in optics and the imme- 
diate problem of overseeing a large number of con- 
tracts which had previously been initiated by the 
Instruments Section. Inasmuch as the new Division 
consisted to a large extent of personnel associated 
with the Instruments Section during 1940 and 1941, 
the reorganization involved few important changes. 

The present Summary Technical Report describes 
the accomplishments of both Division 16 and Sec- 
tion D-3, and covers the principal developments in 
optics made in America during lYorld War II. This 
report should be considered as intermediate in char- 
acter between the detailed contractors’ reports of 
Division 16, to which reference is frequently made 
herein which are complete scientific reports of the 
investigations carried on, and the historical volume 
entitled Optics and Applied Physics in World War 
II, which presents in less technical form the accom- 
plishments of the Division and its contractors, and 
assigns credit to those who took part. 

The contents of the present volume demonstrate 
impressively the great contribution made by the 
optical industry of America and the university opti- 
cal laboratories to the war effort. While less glamor- 
ous than some of the newer fields brought into 
existence during the war, optics nevertheless made 
significant contributions which were by no means 
confined to mere extension or application of optical 
methods or apparatus previously in use. The stress 
of the emergency produced many new optical de- 


velopments, and the genesis of a large proportion 
of these will be found recorded in the following 
pages. 

The science of optics and the optical industry 
have both benefited greatly by the intensive re- 
search which took place during the war. Many of 
the new devices developed under emergency condi- 
tions have contributed and will contribute more to 
our fundamental understanding of optics, and many 
of them will have peacetime applications. New lines 
along which optical research should be directed 
have been made apparent. In particular, the infra- 
red field has benefited greatly, and the art of in- 
frared phosphor development and utilization has 
been elevated to an entirely new level. 

Consideration of the developments in optics, as 
in other fields, emphasizes that, once adequate im- 
mediate defense has been insured, more important 
than having weapons for a possible future war is 
having available a large body of trained personnel 
who can step into any breach that occurs and be 
available to produce the new devices that may be 
needed. 

The Optics Division of NDRC is especially in- 
debted to the chiefs and members of its Sections, 
whose names are listed at the end of this volume. 
They have provided the essential leadership, com- 
bined with scientific knowledge, without which the 
work of the Division could not have been planned 
or completed. 

Geoege R. Haebison 
Chief, Division 16 




CONTENTS 


CHAPTER PAGE 

1 Introduction and Summary by W. E. Forsythe ... 1 

2 Infrared Image Tubes and Electron Telescopes by 

Charles A. Federer, Jr 4 

3 Metascopes by Mary Banning 35 

4 Infrared-Sensitive Phosphors by Franz Urbach and 

Mary Banning 54 

5 Survey of Infrared Sources by George E. Meese . . 70 

6 Ultraviolet Sources and Filters by 

Charles A. Federer, Jr 91 

7 Autocollimators by Mari/ 110 

8 Antiglare Devices by Mary Banning 120 

9 Miscellaneous Optical Developments by 

Mary Banning 127 

Glossary 135 

Bibliography 137 

OSRD Appointees 141 

Contract Numbers 143 

Service Project Numbers 145 

Index 146 



II 



Chapter 1 

INTRODUCTION AND SUMMARY 

By W. E. Forsythe^ 


S ECTION 16.5, National Defense Eesearch Committee 
[NDRC], and the sections that preceded it in this 
work, which were the Illiimination Committee of the 
National Eesearch Council, and Section C-6, Section 
12.1, and Section 16.2 of NDEC, had as their main 
problems methods of seeing road markings and of sig- 
naling without the use of radiation within the visible 
spectrum. This means that ways Avere sought by Avhich 
either ultraviolet [UV] radiation or infrared [IE] 
radiation — each as far he 3 ^ond the limits of the vis- 
ible spectrum as possible — were to be used. The idea 
back of this was, of course, to find some method that 
Avould enable troops, convoys, boats, or airplanes to 
find their way along darked-out roads, shores, or run- 
ways without the enemies being able to see them. 

Two devices by Avhich this could be done by the use 
of infrared radiation had been experimented with even 
before the formation of the Illumination Committee 
in 1940. These Avere the tAA^o infrared telescopes : the 
electronic telescope developed by some of the engineers 
of the Radio C^orporation of America [ECA] Labora- 
tories, and the metascope developed by the physicists 
of the Institute of Optics of the University of Roch- 
ester. Later, special reflecting devices Avere developed 
for use in marking out roads, lanes, ruiiAvays, or 
shores; such devices became visible under irradiation 
by ultraviolet radiation. Similar devices Avere deA^el- 
oped that could be used for this same purpose Avhen 
irradiated by infrared radiation. These several de- 
vices Avill be taken up in turn, first the electronic tele- 
scope. This device is fully described in Chapter 2 of 
this report, but here some of its uses Avill be touched 
upon. 

The first use of the infrared telescope Avas to enable 
the operator of a truck to see his Avay along a darked- 
out road. It Avas demonstrated at Fort Belvoir late 
in 1942 that a truck could be driven along a dark 
road, or through a dark lane through a Avoods, Avith- 
out the truck or its lights being visible to an observer 
50 to 100 yards directly in front of the truck; this 
infrared telescope Avas used by the driver to ^‘see’’ 
the road. Like demonstrations Avere made at several 
other Army camps. 

»Nela Park Laboratories, General Electric Company. 


The use of this infrared telescope for moving tanks 
under blackout conditions was demonstrated, and it 
Avas shoAvn that this could be done even under com- 
plete darkness. Neither the lamp nor the infrared 
source could be seen even when the tank was so near 
to an observer that the heat from the tank and the 
infrared sources Avas quite evident. 

Boats Avere brought to the shore or back to the 
mother ship under dark-out conditions Avith the only 
means of seeing the shore or the ship being the in- 
frared telescope. 

Several demonstrations were made of the use of this 
instrument to pilot a locomotive along a track Avhere 
the engineer depended upon the infrared telescope 
to see his signals. 

There were tAvo criticisms of this device. It was 
difficult to hold the telescope so that good use could 
be made of it and it Avas felt that too much wattage 
Avas required for the infrared sources. As time went 
on, the sensitivity of this infrared telescope Avas great- 
ly inci’eased, and better sources of the infrared radia- 
tion Avere produced. Later, a binocular electronic tele- 
scope was developed that could be carried on the head 
of the user. With this head-home instrument, the 
engineers at Fort Belvoir demonstrated that trucks or 
jeeps equipped Avith special infrared headlights could 
be driven along roadAvays that Avere completely dark 
Avith safety and at a speed at least 25 miles per hour. 

With the use of this head-borne instrument it was 
also demonstrated that a pilot could land an airplane 
on a runway that was marked out with infrared 
sources that could not be seen at all unless the pilot 
had a device of this general type. This was done at 
Lancaster, Pennsylvania, in tests held for the Navy 
and later by some officers of the Navy at the Charles- 
ton Navy Air Base in Rhode Island. 

By the use of this telescope it was shown, at the 
request of the Army, that a target could be seen at 
a distance of about 800 yards in complete dark-out. 
To do this, hoAveA^^ required for ijfie source of in- 
frared radiation twk 60-inch ^archlights, each 
equipped Avith a 3,000-l(satt tui^ten lamp. In front 
of these searchlights was im^frared filter of such 
density and wavelength limlS^hat these searchlights 


REGRADED UJKLA^iE 
ORDER SEC ARMY BY TAG EER 


‘^‘'853 1 



1 


2 


INTRODUCTION AND SUMMARY 


were not visible even for a jjersoii directly in front of 
them and about 100 to 150 yards away. An infrared 
telescope with a Schmidt mirror about 12 inches in 
diameter was used for this job. 

The most popular use that was made of the elec- 
tronic telescope was in the devices called the snooper- 
scope and the sniperscope. With the snooperscope, 
which, with its power pack, source of radiation, and 
infrared telesco]>e, weighed only 20 pounds, the ob- 
server could see a man at a distance of about 100 
yards. The sniperscope consisted of a special infrared 
telescope and a lamp mounted on a carbine and a 
source of power earned on the user’s back, with a total 
weight of 20 to 25 pounds. With such a device, a man 
could be seen at about 100 yards in complete dark- 
ness and good use made of the carbine. Some very 
good reports were received for the uses of the sniper- 
scope in the field, particularly in the Okinawa cam- 
paign. 

A source of infrared radiation, which in general 
consisted of a tungsten lamp in a reflector bulb with 
a filter in front of it, so that it was not visible even 
to a person directly in the line of sight for more than 
50 to 100 yards, could be seen for a very long distance 
(several miles) by the use of this telescope. Thus this 
infrared telescope would enable a pilot in an airplane 
or on a boat to find a position marked with infrared 
sources, even in complete darkness without being 
seen himself. 

The metascope enables an observer to see an in- 
frared source at a distance of several thousand feet, 
and thus it can be used either for locating positions 
marked with infrared sources or for signaling. One 
great advantage of this instrument over the electronic 
telescope is that it needs only a very simple power 
source and also it is not so fragile. It is thus lighter 
and more portable. However, it is much less sensitive 
than the electronic telescope. Because of their ditferent 
characteristics, these two infrared telescopes supple- 
ment each other very well. 

A plan to enable paratroopers to find their jump 
area and then to assemble by the use of these infrared 
telescopes was worked out and demonstrated at Camp 
Mackall, North Carolina. The pilot located a previ- 
ously erected infrared source by the use of the elec- 
tronic telescope; then, after the paratroopers had 
landed, their leader held a flashlight with an infrared 
filter over it to present a source of ijifrared radiation 
that could be located by the men by the use of a 
special very small metascope (Type K). 

There are some devices that will return light al- 


most along the same path by which it reached the 
device. One such device is the triple mirror, an ar- 
rangement familiar to many persons as the common 
roadside reflector button used to mark out the limit 
of the road. Some of these were used in World War I. 
Methods were worked out by the Mount Wilson Obser- 
vatory staff for making triple mirrors. A large number 
were made and used for signal- and range-finding 
work. 

When triple iiiirrors are employed, the observer 
simply holds a light source near his eye and sees an 
answering beam returned to him by the triple mirror. 
For the return beam to be seen from an ideal triple, 
the eye should be as close to the source as possible and 
not farther away than the diameter of the mirror. 

Triple mirroj*s were used to mark out the runway 
on a landing made using a light source near the eye 
of the observer. 'The light source was composed only 
of 2- or 3-candlcpower lamps covered with red cel- 
lophane. This enabled the pilot to see returned beams 
from the triple-mirror runway markers as much as 
7,000 feet away. 

Triple mirrorcs are only one type of autocollimator, 
or retrodirective, reflector. Another type employs a 
Schmidt optical system, with a mirror (convex) as 
the focal surface if visible light is used. For security 
and use with ultraviolet light, the surface is coated 
with a phosphor which emits light when excited by 
ultraviolet radiation. With modification, this system 
may also be used with infrared phosphors. These auto- 
collimators will return light in any direction, over a 
wide range of incidence angles, and should prove very 
valuable to the Armed Forces for either signaling or 
marking the limit of roads or paths through the woods. 
They can also be used to mark out landing beaches. 
Many of these devices were made and tried out for 
various marking and identification purposes. 

Section 16.5 had contracts with a number of labo- 
ratories to help produce better parts for the above- 
mentioned instruments and light sources. Engineers 
in some of these laboratories helped to equip several 
vehicles with spedal sources of infrared radiation; 
in some cases, they installed special power equipment 
so that sources of r adiation could be used consuming 
more wattage than produced by the vehicle’s power 
supply. Also, special light sources were devised and 
built, sometimes on short notice, for various tests. 
These engineers and scientists also acted as consult- 
ants to the Armed Forces and devised some apparatus 
which has not been listed in this summary. 

Contracts were made with six laboratories to do 


INTRODUCTION AND SUMMARY 


3 


special work in an attempt to produce better infrared 
phosphors than were available at the start of World 
War II. Not only was the work done in Europe re- 
produced in this country for the first time, but a new 
and extraordinary series of very sensitive infrared 
phosphors was developed, and the sensitivities and 
ranges of metascopes employing these new phosphors 
were thereby increased substantially. 

In both ultraviolet and infrared detection, commu- 
nication, and ranging systems, optical filters play an 
important part. Contractors foi‘ both Sections 16.4 and 
16.5 tested and explored filter materials of many 
kinds, and resultant improvements in filters for both 
ultraviolet and infrared increased the security and 
ranges of many source-receiver combinations. 


Among the several special problems undertaken by 
Section 16.5 and its predecessors, that of observing an 
airplane when it is almost directly in line with the 
sun and the eye of the observer deserves special men- 
tion. Direct observations were, of course, out of the 
question. Several devices were made and tried out for 
this purpose, but they proved ineffective or too diffi- 
cult to operate. Finally, a phosphorescent material 
that has an upper-limit brightness response after being 
excited by the sun’s rays was employed in a device 
resembling a metascope. This instrument, called the 
Icaroscope, makes it possible to look directly at the 
sun and see an airplane even if it were directly in 
line with the sun and the observer. Several models of 
this device were made and tested. 


Chapter 2 

INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 

By Charles A. Federer, Jr.'^ 


21 INTRODUCTION 

F oe their application in signaling, night firing, 
reconnaissance on land and sea, infrared driving, 
airborne operations, and the like. Section 16.5 under- 
took the investigation of infrared image tubes and 
electron telescopes. Chief contractor for this work 
was RCA Laboratories at Princeton, New Jersey, 
where the infrared telescope had already been studied, 
so the research described herein was carried on to 
develop the image tube and telescope to the point 
where they would have military usefulness. In the 
actual application of the infrared electron telescope 
to military problems, cooperative work was carried on 
among RCA Laboratories, Contract OEMsr-440; the 
University of Pennsylvania, Johnson Foundation, 
Contract OEMsr-1075 ; and the General Electric 
Company, Nela Park, Contract OEMsr-423. 

As applied to military problems, the infrared- 
sensitive telescope operates in the spectral regions 
from 0.8 to 1.0 micron and slightly beyond — that is, 

INFRARED TELESCOPES 

OBJECTIVE LENS IMAGE 



OCULAR 


TUBE 


TO POWER SUPPLY 


SCHMIDT OBJECTIVE SPHERICAL MIRROR 



Figure 1. Two forms of the infrared telescope, one 
employing a refracting objective and the other a 
Schmidt system. 

in the near infrared. Basically, the instrument con- 
sists of an objective for forming an infrared image 
of the scene being viewed upon the sensitive cathode 
of an image tube, the image tube itself, and an ocular 
for viewing the image (in visible light) formed on the 

“Harvard College Observatory. The material in this chapter 
has been prepared and extracted from reference 1. 


fluorescent screen of this image tube. As auxiliary 
equipment, a high-voltage power supply is used to 
actuate the image tube. Figure 1 is a schematic 
diagram of such a telescope, in two forms, one with a 
refracting objective and the other with a Schmidt 
system. 

The essential element is the electron image tube, of 
which Figure 2 is a schematic diagram. The cathode, 
on the end of the glass envelope, consists of a semi- 

IMAGE TUBE 
(IP25) 


ELECTRON LENS 



Figure 2. Schematic diagram of the standardized 
image tube. 

transparent layer of silver which has been treated 
with ox 3 ^gen and cesium. Electrons which leave the 
cathode when radiation strikes that surface are ac- 
celerated through an electron lens system which 
focuses them on the phosphor coating on the other 
end of the tube. Here fluorescence produces the visible 
image. 

As a result of a long series of investigations on 
various types of image tubes, a tube was finally devel- 
oped and put into production as the 1P25 which was 
satisfactory for general infrared work. It was manu- 
factured and used in fairly large quantities during 
the latter part of the war. At the close of the war, 
a high-voltage tube with improved performance char- 
acteristics had been developed and was being put into 
pilot production (see Section 2.3.3). A number of 
other new image tubes showing considerable promise 
had been developed (see Section 2.3.2). 


4 






THE INFRARED IMAGE TUBE 


5 


Telescopes using the 1P25 were developed for many 
military purposes. A signaling telescope, Type C 3 , 
was put into production, as was a gun-aiming instru- 
ment. Several other telescopes were in production or 
on order, but in smaller quantities. Instruments for 
the improved image tubes were also developed. 

Because of the use of image tubes in television pick- 
up devices, serious work on electron imaging began 
at the RCA Laboratories in the early thirties. Initially, 
magnetic electron optical systems were employed, but 
it was soon found that excellent results could be ob- 
tained with electrostatic lenses. By the end of 1935, 
it was possible to demonstrate an operative infrared 
telescope^ with excellent image quality, quite com- 
parable to that of a modern image tube such as the 
1P25. However, the image brightness relative to the 
incident radiation was only 1/500 to 1/1,000 of that 
obtained with the modern tube. A number of these 
early telescopes were built and tested in scientific and 
other applications and their value in maintaining 
military security demonstrated. It was conclusively 
shown that the device could not be used for seeing 
through fog. 

By 1938-1939, the sensitivity had been so increased 
that a car equipped for infrared night driving was 
used in a great many experimental field tests, and 
was demonstrated at Aberdeen Proving Ground. While 
this particular equipment was inadequate, it was 
clearly shown that driving in absolute visual darkness 
was possible. Simultaneously, two filtered searchlights 
with telescopes mounted on them were built for and 
delivered to the Navy for experiments in infrared 
signaling. This equipment was the forerunner of the 
Type C 3 instrument later developed under Contract 
OEMsr-440 and put into production for the Navy. 

Further development of these and other military 
projects will be discussed (see Section 2.5) after the 
technical details of the development of infrared image 
tubes and telescopes and their auxiliary apparatus, 
including some of the problems in designing produc- 
tion models, have been reviewed. 

22 the INFRARED IMAGE TUBE 

2 2 ^ A Standardized Tube 

As the image tube would only be of value during 
the war if it could be produced quickly and in quan- 
tity, it was necessary to choose a single type of rela- 
tively simple construction and to make it possible for 
use in a wide range of devices and situations. 


Type of Tube. There were three general types of 
tube to choose from : 

1 . The first type has a tiniform field between cath- 
ode and screen, which requires close spacing between 
them in order to obtain high definition. Unity magni- 
fication is inherent, a high field is required, and the 
image is erect (leaving final image inverted because 
of the inversion of objective). 

2 . A magnetic lens system is capable of producing 
a sharp true image, but it is in general rotated with 
respect to the image on the photocathode. The mag- 
netic lens system is in general heavy and bulky. 

3. The electrostatic lens system is the type chosen 
for development, for it has the following advantages : 
inverted image (erecting the objective image) ; any 
desired field strength at the cathode; adjustable mag- 
nification. When used with a Schmidt system a curved 
photocathode is naturally provided. One disadvantage 
is that with a refracting objective lens system, an 
optical field corrector must be used to produce the 
proper curvature for the photocathode (see below). 

Magnification. In the image tube, the brightness of 
the reproduced image varies inversely with the square 
of the magnification. This results from the increase in 
the concentration of electrons when the magnification 
is small ; it is better, therefore, to use low magnifica- 
tion in the image tube (such as V 2 instead of 1 ) and 
to make up the difference in the power of the eyepiece 
(such as 10 instead of 5). However, although special 
purpose image tubes might be made, therefore, with 
very small fractional magnifications, various factors 
pointed to V 2 as the best magnification for an all- 
purpose image tube. This magnification does not give 
maximum possible brightness, but does insure such 
results as filling the pupil of the dark-adapted eye or 
furnishing an especially large exit pupil when needed 
for military operations. 

The standardized tube, then, was set at between 
IV 2 and 2 inches in diameter, 4 to 5 inches long, about 
5,000 volts, magnification V 2 by an electrostatic lens. 

Electron Optical Considerations 

In its simplest form, the image tube consists of a 
uniform field between the cathode and main lens, a 
fairly strong main lens, and a constant potential be- 
tween the lens and the fluorescent screen. For a mag- 
nification of V 2 the main lens should be halfway be- 
tween the cathode and screen. 

A rather complete analysis of the electron optics 
of the electrostatic image tube^ had been made prior 




6 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


to the date of the NDRC contracts. While such an 
analysis can by no means give a complete solution to 
the design problem of the image tube, nor can it 
predict completely the performance of a specific image 
tube structure, it is nevertheless helpful in indicating 
the course along which the development should pro- 
ceed. Some of the conclusions are briefly discussed 
here. 

Curvature of the Image Field. When a flat cathode 
is employed, curvature of the field is inherent. To- 
gether with astigmatism, it limits the off-axis defini- 

r ‘ 



LENS SCREEN 

Figure 3. Sagittal and tangential image planes. 

tion. Figure 3 illustrates the shape of the sagittal and 
tangential image planes. 

Curvature can be reduced by a suitable selection of 
weak correcting lenses between the cathode and main 
lens, but the field can be completely flattened only by 
properly curving the cathode. However, as has been 
mentioned, a curved cathode introduces optical com- 
plications with a refracting objective. Therefore, the 
cathode curvature adopted for the standardized tube 
(1P25) is a compromise between light optical and 
electron optical considerations. The radius of curva- 
ture selected was 2.38 inches. With this radius, there 
is still a perceptible loss of definition at the edges of 
the field and some pincushion distortion, but both very 
small in a perfect tube. 

Astiginatism. Astigmatism is reduced somewhat by 
the curvature of the cathode, but with the curvature 
used in the 1P25, neither the sagittal nor the tangen- 
tial image plane is flat. However, the depth of focus 
at the image is quite large because of the very small 
aperture angles of the imaging pencils. 

Associated with this type of aberration are image 
defects which result from mechanical errors in tube 
construction or assembly. Misalignment of the cyl- 
inders and apertures or a departure from roundness 
of some electrodes most commonly cause the lens to 
act as a slightly cylindrical system instead of as a 
truly spherical one. Then the image of a point on the 
cathode is an ellipse with its minor axis in one direc- 
tion for one voltage setting, and at right angles to 
this direction for a second setting. Figure 4 illustrates 


this condition, in which readjusting the voltage to 
sharply focus one set of lines blurs those at right 
angles. However, as manufacturing procedures con- 
tinue to improve, tubes with electron optical defects 
are becoming rarer. 

Chromatic Aberration. This is due to the spread of 
initial velocities of the photoelectrons, and establishes 
the limit of resolution at the center of the image field. 
If is the diameter of the circle of confusion, the 
following approximate equation applies : 

P = zb 2m — , 

E 

where ni is the image magnification, V the initial 
velocity in electron volts, and E the field strength at 
the cathode. Thus E is the primary factor determin- 
ing definition. 

On this basis, the apertures forming the main lens 
should be small, and the limiting resolution obtained 
with the electron lens system arranged as shown in 
Figure 5 A is better than that shown in Figure 5B. 
The main lens in the 1P25 is half the diameter of the 
cathode, which is not the smallest size that could be 
used. The limit to size is set by the point at which 



IMAGE FORMATION WITH POTENTIAL V| 

IMAGE FORMATION WITH POTENTIAL V2 

Figure 4. The elliptical image effect. 

the main lens acts as a field stop, but the size in the 
1P25 is a practical value considering alignment and 
insulation problems of tube production. 

Spherical aberration and coma play a negligible 
part in limiting the definition. 

Electrode Design of the 1P25 

A preliminary analysis of the first-order imaging 
properties of a simplified electron optical system of 
the general type suitable for the image tube suffices to 
establish an outline for the design. This simplified 
system consists of a uniform field between the cath- 
ode and main lens, a fairly strong main lens, and a 
constant potential between the lens and screen. It is 


THE INFRARED IMAGE TUBE 


7 


found that the main lens should be located halfway 
between the cathode and screen for a magnification 
of ^ 2 - Furthermore, the range of aperture-cylinder 



A 



B 

Figure 5 . Large and small aperture lens systems. 


diameters for the main lens and corresponding field 
strength for the secondary lens can be estimated. 

With this as a basis, more realistic models of elec- 
tron optical systems can be laid out, their potential 
distributions calculated, or measured in an electro- 
lytic plotting tank, and ray paths traced through 
the systems. In this way, the fundamental form of 
the image tube can be derived. 

The exact dimensions and arrangement of the elec- 
trodes of the 1P25 are shown in Figure 6. Where the 
overall voltage differs from that indicated, the given 
voltage ratios must be maintained. In production, in 



1^25 EteCTROOe DIMENSIONS AND VOLTASES 

Figure 6. Electrode design of 1P25 image tube. 

general, the length and diameter of each electrode 
can be held quite close to these dimensions. There is, 
however, some variation in spacing, principally in 
that between the cathode and electrode Gi. If the 
potentials to the four electrodes between the anode 
are readjusted, the effect of this variation is in general 
negligible. It is, however, customary in telescope de- 
sign to fix all but one potential, either G 2 or G 3 . 
Because of this, the least incorrect spacing will result 
in incorrect magnification and enhanced off-axis aber- 
rations. The potential distribution and electron paths 
for 2 electrons, (1) off axis with zero initial velocity, 
(2) on axis having initial velocity, are shown in 
Figure 7. 

2.2.3 Photoelectric Cathode Problems 

Photoemission serves as the source of the electrons 
which are imaged by the electron lens system. There- 
fore, the success of an image tube depends upon ob- 
taining an efficient photocathode. 

Work on photocathodes during the NDRC contract 
period was divided into two parts: investigation of 
methods to improve cesium-on-oxygen-on-silver cath- 




8 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


odes for infrared use and to better their reproduci- 
bility; a search for a new photoeniitting surface of 
superior characteristics. The second program was with- 
out significant success, although a wide range of 
materials was studied. 

AVith respect to the first line of investigation, a 
threefold increase in the maximum response to radi- 



Figure 7. Potential design and electron paths in 
1P25 image tube. 


ation at about 0.9 to 1.0 micron was effected. Further- 
more, whereas originally one good surface out of four 
was considered reasonable, the present advance finds 
the experienced laboratory operator capable of pro- 
ducing nine good surfaces in ten attempts. The con- 
tractor’s report^^ describes causes of failure in activat- 
ing surfaces and explains the detection of contamina- 
tion by the color of the treated cathode surface. 

The Cesium-on-Oxygen-on-Silver Surface 

The Alkali Metals. The alkali metals form the basis 
of all the best photoemitters known. In general, the 
long wavelength limit of response increases with the 
atomic weight in the alkali series, and cesium is there- 
fore used for surfaces of high infrared sensitivity. 
In spite of years of research, which include study of 
most of the known elements, no material has been 
found which gives better results, either from the 


standpoint of a longer wavelength limit or of in- 
creased magnitude of the infrared response. 

The pure alkali metals, however, are relatively poor 
photoemitters compared with the same metals used 
to activate complex surfaces of silver and oxygen. 
However, the emission becomes selective, and the 
response curve has several maxima and minima. For 
the present application, the only portion of the cesiuni- 
on-oxygen-on-silver response curve which is important 
is the peak which rises from a minimum in the neigh- 
borhood of 0.55 micron to a maximum between 0.8 
and 0.9 and drops gradually to zero at 1.2 microns 
or even longer (see Figure 8). 

Preparaiion of the Surface. Basically, surfaces of 
this type are prepared by forming on a silver surface 
a thin layer of silver oxide, then exposing the surface 
to cesium vapor, and subjecting the system to heat 
treatment, during which the cesium reacts with the 
silver oxide to form cesium oxide and free silver. 
The final surface then consists of silver, a layer of 
mixed silver and cesium oxide with metallic silver 
interspersed in it, and a bound layer of cesium. The 
cesium itself probably acts as the active centers of 



WAVELENGTH IN M 


Figure 8. Spectral response characteristics of 
cesium-oxygen-silver photoemitter. 

photocmission, and the underlying layer serves to 
provide a suitable environment for binding the cesium 
and also serves to supply replacement electrons for 
those emitted. At present, a satisfactory theory of 
the mechanism of operation of this type of surface is 
lacking, so that attempts to improve the basic surface 
must proceed on an empirical basis. 

Thermionic Emission. Thermionic emission is an 
indication of the state of activation and can be used 
as such during the processing. Some work has been 
done along the line of reducing thermionic emission, 
inasmuch as it is the principal source of background 


liFBTWfTCI ) 


THE INFRARED IMAGE TUBE 


9 


glow ill a well-iiiacle tube. The order of iiiagiiitiide of 
the current density for room temperature is 10“^^ 
ampere per square centimeter, and this current in- 
creases by a factor of 10 for every 20-degree rise 
in temperature. There is some evidence that there 



5000 6000 7000 

WAVELENGTH IN A 


Figure 9. Emission spectra of the two phosphors 

used in image tubes. 

is a limit below which the thermionic emission cannot 
be reduced for a tube having a given infrared response. 
It is also known that by decreasing the response above 
1.0 micron the thermionic emission can be lowered. 

Spectral SensUivUy. A well-sensitized infrared cath- 
ode has a spectral response as shown in Figure 8, but 
some deviation from this curve, particularly at the 
long wavelengths, can be expected in production tubes. 
The absolute response of these surfaces to whole light 
from an incandescent source with a color temperature 
of 2870 K can be as high as 50 microamperes per 
lumen, but usually runs between 25 and 35. If 1 
lumen of light from this source is filtered with Wrat- 
ten 87, the response is cut to about Vs and, when fil- 
tered with 4 millimeters of Corning 2540 heat trans- 
mitting glass, it is about the whole light response. 

2.2.4 Fluorescent Screen Problems 

Of the fairly large class of phosphor materials 
which might be used for the fluorescent screen in the 
image tube, only two have high efficiency along with 
the other characteristies required to make their use 
practical. These are Willemite, which has a very low 
sensitivity to contamination by cesium vapor, and the 
sulfide phosphors, which are very sensitive to cesium 
contamination and must be protected by a film. Fig- 


ure 9 gives the emission spectra of Willemite and zinc- 
cadmiuni-sulfide. 

Both phosphors have been used in image tubes, 
Willemite for the 1P25, and zinc-cadmium-sulfide for 
high-voltage tubes (see Section 2.3.3). Many other 
phosphors have been tested in the course of image 
tube development, including zinc oxide, the tungstates, 
and others, but none has been entirely satisfaetory. 
A perfect phosphor for image tubes would : 

1. Be suitable for vacuum conditions; 

2. Be insensitive to cesium contamination; 

3. Have high efficiency at low current density; 

4. Operate in the voltage range 3,000 to 6,000 volts 
and higher ; 

5. Form a fine-grain, high-resolution screen; 

6. Have a rapid decay time; 

7. Emit light suitable for scotopic vision (not red). 

Willemite has been used for 1P25 image tubes as 

most nearly fulfilling these conditions. Its luminous 
efficiency is from 1 to 3 candles per watt at the volt- 
ages employed. Of great importance is its low sensiti- 
vity to cesium- vapor contamination. It easily makes a 
fine-grain screen, and its color is green or yellow- 
green. But its time-lag characteristics are such as to 
interfere appreciably with signaling speed where an 
instrument is used for this purpose ; this same factor 
also reduces definition' of rapidly moving extended 
objects. The build-up to maximum value requires 0.04 
second; brightness falls to 10 per ('cnt of its initial 
value 0.04 second after excitation is discontinued; and 
the luminescent decay follows an exponential curve so 
that residual glow remains visible a long time after 
intense excitation.^® Signaling speed cannot be much 
more than five words per minute. 

The sulfide phosphors are somewhat more efficient 
than Willemite, although at 1P25 voltages the differ- 
ence is unimportant. When contaminated, sulfide phos- 
phors lose most of their efficiency; protecting alumi- 
num or other metallic films absorb a large percentage 
of the bombarding electrons at low voltages, but at 
high voltages this protective film is very satisfactory. 
Various shutters and removable protective barriers 
have been tried to reduce contamination but without 
practicable success. 

The phosphors are prepared as exti emely fine pow- 
ders with paiticle sizes of the order of 1 micron. Pre- 
paration is described in the contractor’s report.^"^ For 
Willemite used at 5,000 volts, the screen density is 
about 1 milligram per square centimeter; for 20,000 
volts, it is 21 / 2 . A sulfide screen should have about the 
same density. 




10 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


2 2 ^ Construction of the 1P25 

The contractor’s report^® outlines the construction 
and activation procedure in sufficient detail to permit 
an experienced laboratory worker actually to build an 
image tube which would be a laboratory version of 


Figure 10. The 1P25 image tube. 

the production 1P25. The procedure outlines would 
not, of course, be feasible for production purposes. 

Cleanliness is the most important single factor con- 
tributing to the success or failure of the image tube. 
All the parts must be thoroughly cleaned before as- 
sembly and kept clean through all subsequent opera- 
tions. 

Figure 10 is a photograph of the complete image 
tube; Figure 11 shows its component parts; and Fig- 
ure 12, its dimensions. 

2 . 2.6 Performance of the Image Tube 

Resolution 

In rating experimental tubes for television, it is the 
practice to specify the definition as the maximum 
number of black and white horizontal lines which can 
be resolved in a rectangle with a 3x4 aspect ratio 
whose diagonal is equal to the diameter of the image 
area. Ambiguity exists in that the percentage contrast 
between the black and white lines is not specified, and 
the minimum detectable contrast by eye depends upon 
the brightness of the image. Under brightness condi- 
tions giving approximately maximum resolution, how- 
ever, the results are fairly reproducible. Under these 
conditions, a difference in brightness of 1 or 2 per 
cent is usually taken as the limiting value at which 
the lines can be resolved. The method gives a conven- 


ient and rapid way of rating tubes by simply project- 
ing a pattern of calibrated wedges made up of bundles 
of lines onto the photocathode and observing the re- 
produced image. 

Occasionally, the definition is specified in terms of 
the total number of black lines which can be resolved 
along a diagonal. Ji N is the resolution by the rating 
procedure outlined above, the definition in terms 
of black lines along a diameter is 

„ = ® = 0.83iV. 

Sometimes it is convenient to specify the number of 
lines per millimeter which can be resolved at the 
photocathode. For the 1P25, the conversion in this 
case is 

^ ^ 1 nnoAT 

For experimental image tubes, a resolution of iV^ = 
450 has been generally considered as acceptible. Ac- 
tually a well-constructed tube, correctly focused and 
shielded from external fields, will give better than 
twice this resolution. The limiting definition as de- 
termined by the chromatic aberration for a tube of 
this type is around N = 2,000 or 2,500 for an infrared 


Figure 11. Components of the 1P25 image tube. 

image near threshold. A whole light image will give 
a much lower limiting resolution because of the higher 
initial velocities of the emitted electrons. 

For viewing an extended object, N — 450 gives 
nearly all the resolution that can be perceived with 
the dark-adapted eye, when combined with a reason- 
ably high-powered eyepiece (e.g., X8 to XIO) and a 
practical infrared illuminator. Where the tube is used 
in a signaling instrument close to threshold, the de- 
finition may be much lower without impairing the 
usefulness of the instrument. 






THE INFRARED IMAGE TUBE 


11 


Curvature of the Focal Surface. The resolution, in 
general, will not be uniform over the cathode area be- 
cause of the curvature of the image field. The circle 
of maximum definition can be shifted by the focusing 
voltage. With a well-constructed image tube, the volt- 



age is usually adjusted so that the circle of maximum 
definition has a diameter of perhaps a quarter of an 
inch. Under these conditions, if the definition at the 
■ center is W = 450 lines, the definition at the edge (i.e., 
a circle 1 inch in diameter) will be about 300 to 350 
lines. However, if the lens dimensions are incorrect, 
or the voltages not properly chosen, the definition may 
be much poorer at the edges even though it is sharp 
in the center. 

A correctly built and adjusted image tube will show 
very little distortion, usually only a slight amount of 
pincushion distortion being noticeable at the edges of 
the image. Like loss of definition at the edge of the 
image, distortion increases rapidly with incorrect elec- 
trode spacings or incorrect electrode voltages. 

The hemisphere on the screen end of the image tube 
(Figure 10 ) tends to correct pincushion distortion 
when used with a low-power eyepiece. This is only one 
of the functions of this hemisphere. In addition, it 
increases the effective brightness of the image for the 


fraction of the screen that is in optical contact, and 
it adds approximately two-power to the ocular without 
appreciably reducing the practical eye aperture of the 
system. 

Voltage Focusing. The resolution will, of course, be 
lowered if improper focusing voltages are applied. 
Figure 13 shows the effect on resolution of a departure 
from the optimum focusing voltage. Curve No. 1 is 
for 6^2 ^^sed as the focusing electrode and curve No. 2 
for focusing with G^. In each case, the voltages on the 
remaining electrodes are assumed constant. For any 
value of 6^2 over a considerable range, can be re- 
adjusted to obtain sharp focus at the center. However, 
unless Gg and Gg are correctly adjusted, the magnifi- 
cation will be incorrect and the off-axis aberrations 
large. 

Sensitivity 

The ratio of the amount of whole light on the cath- 
ode to that emitted by the screen is a measure of the 
sensitivity of the image tube; ‘‘conversion’’ has been 
adopted to express this sensitivity. Conversion is de- 
fined as the ratio of the number of lumens emitted by 
the fluorescent screen on the side from which the im- 



80 85 90 95 100 105 110 115 120 129 

PER CENT OPERATING VOLTAGE 

Figure 13. Resolution versus electrode voltage setting. 


age is viewed, to the number of lumens falling on the 
cathode. The output from the screen is measured with 
a photronic cell having a response characteristic ap- 
proximating that of the eye. The cathode is illumina- 
ted by an incandescent source with a color temperature 
of 2870 K. The conversion is easily measured inas- 
much as it is independent of the area of cathode illu- 
minated and the magnification of the image tube. 





12 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


The conversion deals with total light flux to and 
from the tube. However, inasmuch as the fluorescent 
screen closely follows LamberUs law, a knowledge of 
the magnification is all that is necessary to transform 
conversion into units of brightness and intensity. For 
the 1P25, where the magnification is V 2 , the bright- 
ness of the screen B in candles per square foot in terms 
of foot-candles incident on the photocathode is 

4 

B = -IC, 

TT 

where C is the conversion. 

Infrared Conversion. An exact statement of per- 
formance in the infrared is difficult as it involves an 
integration of the product of the spectral response 
curve and the radiation distribution curve. However, 
there are a number of approximate methods which can 
be used which are satisfactory from a practical stand- 
point. One method is to specify the measured whole 
light conversion and a filter factor giving the amount 
by which the response is reduced by the filter in ques- 
tion. The disadvantage of this procedure is that small 
differences in the long wavelength response between 
image tubes cause the filter factor defined in this way 
to vary widely from tube to tube. 

A more practical specification is to determine the 
filter factor giving an attenuation in the neighborhood 
of 10, either by direct measurement with an image 
tube having a standard spectral distribution curve or 
by comj)uting it from the filter transmission curve 
and the standard response curve. Using this filter, the 
luminous flux output from the image tube being test- 
ed is measured with a source delivering 1 lumen (un- 
filtered). This value is then multiplied by the filter 
factor giving a nominal whole light conversion. The 
nominal whole light conversion will equal the meas- 
ured whole light conversion if the tube has a nearly 
standard spectral response, but may differ consider- 
ably in many cases. However, it has been found ex- 
perimentally (over the range of practically useful 
filter factors — from 5 to 15) that in nearly every case 
the nominal whole light conversion divided by the 
filter factor will give a good approximation to the 
tube performance. 

The expected conversion can be easily computed 
from the data on photocathodes and the performance 
of the fluorescent screen. For the purpose of making 
this estimate, assume that the photosensitivity is 20 
microamperes per lumen and the screen efficiency 8 
lumens per watt. One lumen on the cathode will pro- 
duce 0.08 watt at 4,000 volts. The luminous output. 


and hence the conversion, will therefore be 0.64. Meas- 
ured conversions for useful tubes run between 0.25 
and 1.5 lumens/lumen. 

The light output from the image tube is closely 
proportional to the light on the photocathode over the 
useful range of the tube. At very high light levels, the 



VOLTAGE IN KV 

Figure 14. Conversion versus voltage. 

phosphor begins to saturate and the light output 
ceases to increase as rapidly as the incident light. 

The conversion is a function of the overall voltage 
on the tube. The variation of conversion with voltage 
for a number of image tubes is shown in Figure 14- 
With Willemite screen, the increase in conversion is 
approximately proportional to 

Contrast 

The contrast in the reproduced image depends upon 
a large number of factors. Leaving out such effects as 
optical scattering in the objective and ocular as not 
pertinent to a discussion of the image tube itself, 
there are other possible optical effects : scattered light 
in the electron lens structure being reflected back 
onto the cathode, regeneration between fluorescent 
screen and cathode, and internal reflections in the 
hemisphere. Tests show that reflection back to the 
cathode from the internal structure and optical feed- 
back between screen and cathode are quite small. In- 
ternal reflection in the hemisphere is not negligible 




THE INFRARED IMAGE TUBE 


13 


and several per cent of the light from the screen may 
be returned to other points on the screen. A nonre- 
flecting coating would avoid this loss in contrast. At 
low light levels, the background due to two types of 
electrical effects is tiy far the greatest cause of loss of 
contrast. 

Thermionic Emission. As already described, ther- 
mionic emission amounts to ampere per square 

centimeter at room temperature. The screen bright- 
ness due to this current is sufficient to be observable 
with the well dark-adapted eye. However, in the prac- 
tical operation of the infrared telescope, it only inter- 
feres with contrast when the image brightness is ex- 
tremely low, close to visual threshold. With the pres- 
ent metliod of measuring the sensitivity of signaling 
instruments, thermionic background can be an impor- 
tant factor. Where the voltage is higher or the magni- 
fication lower than that used in a ir25, the thermionic 
background becomes a matter of prime importance. 
The thermionic background varies considerably among 
different tubes, even when the spectral responses are 
quite similar. There is some evidence which indicates 
that the minimum thermionic background for tubes 
which have a high response at long wavelengths will 
be higher than for those which have a smaller re- 
ponse, even though the whole light response of the 
two classes of tubes are the same. 

Field Emission. When there is a high potential 
gradient at a metal surface, electrons will be drawn 
out of the material even when its temperature is too 
low for any appreciable thermionic emission. This 
field emission or cold discharge increases exponenti- 
ally with the field strength, and increases as the work 
function decreases. Since cesium is used in the cath- 
ode activation, the work function of the metal surfaces 
of the electrodes in the tube is reduced by its presence ; 
accentuating the tendency for field emission from 
them. Furthermore, if there is any roughness of the 
metal surfaces, high field strengths will exist at points, 
high places, and sharp edges. 

Because of field emission, the metals used in an im- 
age tube must be such that, even in the presence of 
cesium, they will have as high a work function as pos- 
sible. The free cesium left in the tube after activation 
should be reduced to a minimum. The spacing be- 
tween electrodes, particularly those having a large 
potential difference between them, should be as great 
as is consistent with the electron optical design. All 
metal surfaces should be smooth, preferably polished, 
and sharp edges must be avoided. 

The highest gradient in the 1P25 is at the main 


lens, between electrodes and G^. Therefore, these 
are formed in such a way that the metal edges are 
turned back away from the high-field region. The sec- 
ondaiy lenses which are formed by the other electrodes 
do not require as large potential differences, and the 
cylinder edges of (tj, G^, and G^ do not cause trouble. 
These edges are smoothed and rounded as much as 
possible. In the case of the experimental lP25’s built 
during the contract period at the EGA Laboratories, 
the entire structure was polished electrolytically. 

Even with these precautions, cold discharge is not 
eliminated and it sets an upper limit to the voltage 
which may be employed. Figure 15 shows the varia- 
tion of background with voltage for a number of typi- 
cal experimental tubes. It will be noticed that the 
tubes differ quite markedly in their performance in 
this respect. By selecting tubes, those can be found 
which can be operated much above their present rat- 
ing, even to overall voltages of 10,000 or 15,000 volts. 



Figure 15. Background glow versus voltage for 
1P25 and aperture image tube. 


This, in general, is not advisable because even these 
selected tubes may develop cold discharge in the course 
of their life. 

Dust or particles settling on the electrodes during 
construction and assembly of the tube can completely 
nullify the care taken in smoothing and polishing the 
electrodes. Such particles will constitute points about 
which the field strength will be high, and therefore 
cold discharge is likely to originate from them. When a 
completed tube shows bad cold discharge upon initial 
test, it can frequently be greatly improved by running 
it at considerably over voltage for a short period. 


«*wfiinTrTTfS i. ^ 


14 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


An investigation was made of the effect of a small 
aperture placed beyond the main lens at the point of 
minimum diameter of the electron ray bundles. The 
effect of such an aperture is shown in Figure 15. It 
will be seen that while the rate of rise of background 
is reduced at higher voltage, the background level is 
still high. Nevertheless an aperture of this type may 
be of advantage for some applications of the image 
tube (such as the sniperscope) but not for others 
(threshold-point source sensitivity). 

23 EXPERIMENTAL IMAGE TUBES 

^ Single -Voltage Tubes 

One of the objects of the work under Contract 
OEMsr-440 was to develop a series of lightweight 
telescopes having a long operating life. The multiple 
voltage of the lP2o limited the reduction in size be- 
cause of the consequent requirement of a voltage di- 
vider load on the power supply. Therefore, consider- 
able work done on single-voltage image tubes. 

To insure maximum usefulness, the design was 
l)ased on a tube of the same external size and shape 
as the 1P25, so it could be substituted for it in the 



Figure 16. Components of the single-voltage image 
tube (Type U-41). 


then existing telescopes, and could be used in new in- 
struments with smaller, longer-lived power supplies. 

Where the requirements of V 2 magnification and 
particular size have to be met, the electrons have to 
be accelerated by secondary lenses before the main 
lens, not only to correct for distortion and field cur- 
vature, but also to obtain a focus. A series of unit 
lenses, consisting of pairs of coaxial cylinders at cath- 
ode potential with positive cylinders surrounding the 
spaces between them, gave good images in laboratory 


models, but were entirely unsuited for production be- 
cause of very close tolerances. 

The U-41 Tube 

The method finally adopted involves a radically new 
type of electron lens structure, although the main lens 
is essentially the same as the lens in the 1P25. As 
shown in Figure 16, the secondary system is formed 
by the field between a structure of tapered lateral 
strips and a surrounding positive cylinder. The di- 
mensions of this tube (U-41) are given in Figure 17. 
The inner and outer electrodes are self-supporting 



UMPOTENTIAL IMAOE TUBE- ELECTRODE ARRANCEMENT AND DIMENSIONS 

Figure 17. Electrode design of the U-41 image tube. 

units requiring insulating beads only at the ends of 
the cylinders to space them. 

A field plot of the potential distribution in planes 
which include the tube axis is shown in Figure 18. 
The dotted equi potentials show the distribution be- 
tween the strips and the solid lines show that at the 
center of the strips. It is evident that the two distribu- 
tions do not differ significantly, except close to the 
electrodes, well outside of the electron paths. The aber- 
ration produced by the difference is well below the 
other aberrations of the electron optical system and 
does not produce a measurable degradation of the 
image. 

Since these tubes are designed for a low-power, 
high-voltage supply, the avoidance of electrical leak- 
age is very important. Leakage may take one of two 
forms, either due to conductivity over the insulating 
beads or cold discharge between the inner and outer 
electrodes. Both of these are reduced by keeping the 
structure and components clean, and by keeping free 
cesium in the tube to a minimum. The metal elec- 


EXPERIMENTAL IMAGE TUBES 


15 



trodes should also be polished. By proper attention to 
these details, it was found possible to build experi- 
mental tubes with an effective resistance as high as 
100,000 megohms at the operating voltage, 4,000. 

A number of these tubes were built in the labora- 
tory and used in single-voltage telescopes. The U-41 
tube was not put into production. 

The SU-2 Tube 

Another, very simple single-voltage image tube was 
designed to be used in converting the Type A meta- 
scope (Chapter 3) into an electron telescope. This 
tube employed unity magnification and its length-to- 
diameter ratio was so chosen that a simple two-cylin- 
der lens in conjunction with a cathode having suitable 
curvature would yield an image sufficiently free from 
distortion and curvature to be useful. A number of 
samples were built, but the tube was not put into pro- 
duction. 

2 3 2 Low-Magnification Tubes 

In addition to the V 2 magnification of the 1P25, 
two other ranges of magnification, about Vs and Vso? 
were investigated. As already pointed out, the bright- 
ness of the reproduced image of a given object in- 
creases inversely with the square of the magnifica- 
tion. This method of increasing the brightness can be 
continued until the aperture ratio of the electrons 


leaving the main lens becomes so large that spherical 
aberration makes the image unusable. However, the 
design of the infrared telescope sets a practical limit 
to the magnification which is much higher than this 
fundamental limiting value. 

Tubes of Magnification. Some tubes with 3- 
inch cathodes and %o magnification were experi- 
mented with and found to produce fair images but 
with definition of only about 100 lines. It was, of 
course, essential to cool the cathode to a low tempera- 
ture to reduce thermionic emission, but even then the 
background was fairly high. After work on several 
tubes of this type, it was decided to concentrate on 
tubes of higher magnification. 

Tubes of Vs Magnification. A series of tubes having 
the same cathode size as the 1P25 but with magnifica- 
tions of % to Vs was constructed, and images re- 
solving 300 lines obtained. With dry-ice cooling of the 
cathode, the screen was reasonably dark with no inci- 
dent light, and the expected gain in brightness over 
the 1P25 was obtained. This type of tube, however, 
was superseded by a high-voltage, low-magnification 
tube, which was built in a larger blank having a 3- 
inch cathode (see below). 

233 High-Voltage Tubes 

The lowering of the portability requirements for 
certain types of instruments, and improvements in 


16 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


power-supply design brought luiicli higher voltages 
into the range of practicability. For a AVilleniite 
screen, the light output varies approximately with 
% and for the sulfide phosphors nearly as V-. Fig- 
ure 19 shows the screen efficiency in millilumens per 
microampere for a nund)er of image tubes with screens 
of each type. In high-voltage tubes, sulfide phosphors 
may be employed, for the 2,000 to 4,000 volts lost in 
an aluminum film of sufficient thickness to protect 
the screen is more than offset by the efficiency gain 
in using the sulfide phosphor and due to the light re- 
flected back by the metal film. 

High voltage also results in improved resolution, 
due in part to increased field strength at the cathode 
(reducing chromatic aberration), and in part to the 
greater depth of focus. 

The MA-4 Tube 


Although the electrodes and assembly of the 1P25 
might have been redesigned to stand the higher volt- 
age, it was decided to expedite getting the high-volt- 



Figure 19. Screen efficiency versus voltage (Type 
MA-4 image tube). 


age tube into production by using an initial lens sys- 
tem identical with the 1P25, supplemented by a 
series of anode lenses over which the high voltage is 
distributed. As shown in Figure 20, the structure and 
blank up to and including the pin circle were made 
identical to the 1P25, and the midtiple-anode lenses 


take the form of flanged cylinders sealed into a tub- 
ular neck carrying the fluorescent screen. 

Eilectron optically, the multiple-anode image tube 
behaves somewhat differently from the 1P25. The 
weak lenses form a real electi’on image of a virtual 
object, the latter being the image formed by the main 



Figure 20. Electrode design of Type MA-4 image 
tube. 


lens. In order to bring the real image into focus, elec- 
trode G^3 must be somewhat more positive than in the 
1P25. The magnification of ^ is maintained in spite 
of the increased anode length; pincushion distortion 
is reduced, while increased field curvature is compen- 
sated by greater depth of focus. Willemite fluorescent 
screens may be used in these tubes, but considerable 
advantage is gained if a sulfide is employed. 

Operating Voltages. The operating voltages used on 
the MA-4 are: Cathode, 0; G^, 10; G^, 100; G^, 800 
to 1,000, adjustable; 4,000; A^, 8,000; A3, 12,000; 
A4, 16,000. As with the 1P25, the cold-discharge 
points must be removed by overvoltages before the 
tube can be successfully operated. 

The Type MA-4 tubes give a useful conversion fac- 
tor from 5 to 8 times that of the 1P25, and a number 
of these tubes were built and used in experimental in- 
struments. At the close of the war, a procurement 
order for a quantity of these tubes had been placed 
by the Armed Forces. 

Low-Magnification Tube 

As mentioned in Section 2.3.2, a high-voltage, low- 
magnification tube was developed. It employed the 
multiple-anode structure, made of aluminum, care- 
fully smoothed and polished (Figure 21). The fluo- 
rescent screen is an aluminum-protected layer of zinc- 
cadmium-sulfide. Instead of 5,000 volts, 20,000 were 
used, and an excellent image, high definition, and a 
brightness gain of 50 to 100 times over the 1P25 was 
obtained. Work with this type of tube (MA-6) and 
a telescope incorporating it was in progress as the 
contract terminated. When this tube was used in 


IMAGE TUBE POWER SUPPLIES 


17 


coiijuiictioii with a large Schmidt optical system, it 
was possible to exceed the sensitivity of the dark- 
adapted eye when whole light from an incandescent 
source was employed. 

Image Tube Research 

Lines along which further work might well be done 
include : 

1. Photocathodes may eventually be improved, but 
independent research over a number of years, on the 





Figure 21. High-voltage low-magnification image 
tube (Type MA-6). 


part of the British, the Germans, and ourselves, has 
produced cathodes almost identical both in spectral 
response and in sensitivity. 

2. For the low- voltage tube, a high-efficiency phos- 
phor with a short persistence time, insensitive to 
cesium contamination, is badly needed. The fluores- 
cent screens used now in high-voltage tubes leave 
much to be desired with respect to particle or aggre- 
gate size, uniformity, and resolution. There are many 
known phosphors which might be suitable for image 
tube work, but which have not been studied because 
of insufficient time. 

3. Revision of the basic electron optical design of 
the 1P25 should aim toward elimination of cold dis- 
charge and to permit more accurate construction and 
alignment. 

4. The question of retaining the multiple anode 
on high-voltage tubes should be considered — higher 
field strength at the cathode resulting from omission 
of the multiple anode would reduce chromatic aberra- 
tion. On the other hand, such an increase in field at 
the cathode may add to background glow. 

5. Much remains to be done as far as image bright- 
ness is concerned. It is felt that by increased magni- 
fication, with high voltage and an antimony photo- 
cathode, it might be possible to build an instrument 
which is much moi’e sensitive than the eye for night 


vision. Other methods of increasing brightness, such as 
phosphors, photocathode cascading, secondary emission 
multiplication, and the image amplifier, should be 
thoroughly explored. 

G. The military applications of image tubes employ- 
ing the middle infrared and far infrared regions are 
numerous. It has not yet been possible to extend the 
sensitivity of photoemitters beyond 1.4 or 1.5 microns, 
and there is no evidence to indicate that they can be 
extended appreciably into the infrared. Therefore, an 
image tube operating in the intermediate or far in- 
frared must be based on something other than the 
external photoelectric effect. Thermal and far-infrared 
imaging, in particular using a ‘^S'elocity selectioiP’ 
tube, may become very important. These proposals are 
discussed in the concluding portion of the contractors 
report.^^ 

24 IMAGE TUBE POWER SUPPLIES 

All of the electron image tubes described in this 
chapter require a power supply giving a rather high- 
voltage output. Portability requirements necessitated 
small batteries with a minimum operating life if the 
instrument was to be practical. Fortunately, at 4,000 
to 6,000 volts, the current demand is very small — the 


K* 901614 

veWATOR XT- 4284 



Figure 22. Basic circuit of vibrator power supply. 


image tube itself requires only a fraction of a micro- 
ampere, while the voltage divider needed to supply 
the various focusing electrodes of the 1P25 uses under 
50 microamperes. The power output, therefore, is of 
the order ot oi a watt. 

Component Parts 

The Converter Unit 

The only practically available means for converting 
the low battery voltage is a vibrator-transformer- 
rectifier combination, such as illustrated in Figure 
22. It differs from the conventional vibrator power 
units used in battery-operated radios in that the trans- 
former is resonated by tuning the primary, so that a 


riS&TiRKThiG 


18 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


very high voltage appears across the primary each 
time the vibrator interrupts the current. A corre- 
sponding voltage peak is thereby induced across the 
secondary. A filter with the time constant R X C 
large compared to the vibration period in the output 
circuit smooths out the voltage pulses after rectifica- 
tion. Where the divider load is, for example, 100 
megohms, the capacity required is about 100 /x/xf. 

Vibraior. Due to the rather stringent requirements 
of load, size, and weight, special consideration had to 
be given to the selection of components. For ease of 
procurement, a standard type of vibrator was used, 
one which required minimum power, of the order of 
0.2 to 0.3 watt. The rate was 100 interruptions per 
second, with ‘^on’^ time approximately equal to “off*’ 
time. 

Transformer design makes use of the relatively 
high-voltage peaks across the transformer due to the 
sudden collapse of the magnetic field when the pri- 
mary circuit is broken. A rough approximation of the 
rate of collapse was based on the assumption of 100 
vibrations per second, 0.005-second time of contact, 
3-volt battery voltage, 3-ampere peak current, 4,000- 
volt peak voltage. The contractor’s report^^ explains 
the method of concluding that the primary should have 
100 turns and the secondary 13,000 turns. Higher 
voltage can be obtained by increasing the primary 
flux — by increasing the core size. However, a com- 
promise in transformer size had to be adopted for 
each particular instrument design. The S-4 trans- 
former, complete with filament winding for the rec- 
tifier, weighs 3 ounces. 

Rectifier. This presented a difficult problem for 
portable units because of the lack of a rectifier suitable 
from the standpoint of physical size and filament cur- 
rent. Consequently, it was necessary to develop new 
rectifiers for the purpose. The first type is a thermionic 
rectifier which requires a heater current of 50 milli- 
amperes at 1.5 volts and will deliver up to 100 micro- 
amperes at 5,000 volts. This tube was designed to use 
standard miniature receiving tube parts as far as 
possible and to present a minimum of manufacturing 
problems. It has since been put in rather large produc- 
tion with the designation, RCA 1654. 

A second type of rectifier, the KR31 (Figure 23), 
was developed for this contract for use primarily with 
the single-voltage image tube. It is designed to occupy 
essentially the same space as the RCA 1654, but dif- 
fers from the latter in that it requires no heater 
current. This greatly reduces the load on the primary 
batteries when the rectifier is used intermittently, as 


in the case of the impulse-power supply described 
later, and simplifies the insulation problem in the 
case of the voltage multiplier, also to be described. 

The rectifier depends for its action upon a gas dis- 
charge in helium at approximately 0.5 millimeter 
pressure. (Other gases such as neon may be used.) The 
cathode is simply an aluminum cup or disk. The anode 
is a nickel rod or tubing, over which is fitted a woven 
Fiberglas sleeve. The Fiberglas is heated to remove 
the organic lubricant with which it is impregnated. 
The entire nickel wire must be covered with the Fiber- 
glas; the sleeve fitting down over the Dumet wire 
seal on one end and closed by fusing the glass at the 
free end of the rod. 



The peak inverse voltage of the KR31 is 6,000 volts 
and the forward breakdown voltage between 300 and 
600 volts. The current-carrying capacity depends upon 
the operating conditions. In the application for which 
it was designed, the average current is under 10 
microamperes, but the peak current may be several 
milliamperes. Under these conditions, the tube has a 
long operating life. Complete life tests have not been 
made, but tubes have been operated for periods of 100 
hours with no observable change in performance. 

The Voltage Divider 

A high degree of stability of the overall voltage is 
not essential but the ratio of voltages on the various 
electrodes must be maintained, as was pointed out in 
the section on image tubes. Since the overall voltage 
varies considerably as the batteries discharge and since 
the instruments may be subjected to wide ranges of 
temperature, behavior of the components of the volt- 




IMAGE TUBE POWER SUPPLIES 


19 


age divider as regards temperature and voltage was 
a matter of considerable concern. As it is not always 
possible to maintain the proper voltage ratios over 
the range of temperature and voltage encountered in 
the field, occasional refocusing is necessary. The vari- 
ations can be greatly reduced, however, by proper 


-|-G0 C. Divider No. 2 remains in focus from — TO C 
to +75 C. Therefore, using storage battery supply 
and selected components for the voltage divider, it is 
possible to build an instrument which will not require 
electrical focusing in the field under the range of 
conditions usually encountered. 



Figure 24. Temperature characteristics of composite voltage dividers. 


choice of components in order to balance the charac- 
teristics of the components. 

All of the available high resistances (50 megohms 
or more) show considerable change of resistance with 
voltage. Using dry cells as a source of power, a 2 to 1 
change in overall voltage may be encountered from 
start to end point. Under these conditions, it is im- 
possible to maintain focus without adjustment since 
a 50 per cent change in voltage represents a change 
of about 5 per cent in resistance of the best resistor. 
Therefore, unless compensation could be provided, it 
is necessary to refocus as the batteries deteriorate. In 
the case of storage battery supply, about 10 per cent 
change in voltage may be expected over the operating- 
life. This produces a negligible change in resistance 
of the IKC Type MV resistor and no refocusing is 
necessary. 

Most resistors have a high temperature coefficient, 
so it is necessary to select components which either 
have the same coefficient so that the ratio remains 
the same over the temperature range, or which have 
coefficients which tend to compensate for each other. 

In making up a divider, many combinations of 
resistors tending to compensate are possible. Figure 
24 shows the characteristics of two combinations. In 
both cases, the voltage remains essentially con- 
stant over the entire temperature range, the small 
variations being in such a direction as to compensate 
for the variation in G^. With divider No. 1 adjusted 
for focus at 20 C, the voltage on G^ remains in the 
region of good focus over the range from — 10 C to 


2 2 Power Supply Units 

Power Supply. This power unit was used with 
the Type Z binocular, the Type L monocular, and 
other general uses. It was required to deliver 4,000 
volts to a 100-megohm divider, with a drain of 1 watt 
from a 2-volt battery. It employed essentially the con- 
verter unit already described and its transformer 
weighed only 3 ounces, as already mentioned. 

Power Supply. Figure 25 shows this interesting 
modification of the vibrator power supply. This ar- 



rangement is similar to the conventional voltage- 
doubler circuit except that the two halves of the 
doubler are brought out separately. In this way, it 
is possible to place a voltage divider across one side 
without disturbing the other. In the vibrator supplies. 




20 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


the a-c wave is iionsymmetrical, being in the nature 
of a damped oscillation, so that in the circuit shown, 
the voltage across the high-voltage section, which is 
determined by the first loop of the wave, is about 

4.000 volts while the voltage in the opposite section, 
determined by the second or negative loop is about 

1.000 volts. Therefore, by putting the voltage divider 



O 100 200 300 400 500 

RESISTANCE IN OHMS 


Figure 26. Voltage control of Type S-2 power supply. 


across only the low-voltage section, the desired low 
voltages may be obtained without loading down the 
high-voltage section. 

S-2 Power Supply. Another very interesting feature 
of this circuit is the fact that by introducing resistance 
in the tuned primary circuit, the damping of the cir- 
cuit is increased, which tends to decrease the second 
or negative loops and thus the low voltage without 
appreciably affecting the high voltage. This action is 
shown in the curves in Figure 26. Thus, we have a 
means of varying the focusing voltage by an element 
in the primary circuit, which is a great advantage 
from the standpoint of electrical design. This circuit 
was used in the preliminary C3 telescopes. 

Impulse-Power Supply. In the single- voltage tube, 
the only load on the power supply is the actual photo- 
current and leakage. By careful design, the entire 
load resistance can be made as high as 10^® ohms. 
Using a relatively large capacity on the output, the 
time constant of the circuit can be made several sec- 
onds so that a quite infrequent charging of the cir- 
cuit is required. For this purpose, an interrupter was 
designed consisting of an electrically driven balance 
wheel having a period of about V2 second. The design 
was such that the transformer primary is open most 
of the time and is closed for a short time to allow 
the current to build up and immediately open. In this 
way, the drain on the battery is extremely small, the 
supply operating for as long as 50 hours on a single 
size-D flashlight cell. The interrupter, the 1-ounce 


transformer designed for the purpose, and the KR31 
rectifier are shown in Figure 27. 

S-5 Power Supply. The Type MA-4, high-voltage 
image tube brought up some special problems in 
power-supply design. The overall voltage required is 
in the range of 15 to 20 kilovolts and in addition, 
voltages of 10, 200, 1,000, 4,000, 8,000, and 12,000 
are required. These intermediate voltages, particularly 
the 4,000 volts and over, are difficult to obtain effi- 
ciently by conventional means because of the relatively 
large power which would be wasted in a voltage 
divider of sufficiently low resistance to be stable. Also, 
as was pointed out before, it is possible to obtain 
higher voltages from the previously described power 
supplies only by increasing the flux in the transformer. 
This in turn can be accomplished only by increasing 
the primary power, necessitating larger transformers 
and batteries. Lastly, if a conventional power supply 
is used, a rectifier tube capable of withstanding 20 to 
30 kilovolts inverse voltage would be necessary. This 
type of rectifier is not available in small size and 
low filament power. Consequently, a cascade-type 
(voltage adding) power supply was designed which 



Figure 27. Components of impulse-power sui)i)ly. 

overcame most of the objections and automatically 
provided the necessary four steps of high voltage 
without a voltage divider. 

A schematic diagram of the S-5 power supply mak- 
ing use of this circuit is shown in Figure 28; a photo- 
graph of the S-5 unit is Figure 20. x\s can be seen, 
this supply is made up of four rectifiers which are 
essentially in parallel for alternating current. The 
d-c voltages developed across the rectifiers, however, 
are added by means of the resistors which connect the 
cathode of one rectifier to the plate of the next and 



IMAGE TUBE POWER SUPPLIES 


21 



Figure 28. Circuit of voltage-qiuidiTi})ler power siii)ply (Type S-5). 


thus place all the rectifiers in series for direct current. 
These resistors offer much higher impedance to the 
alternating current than do the capacitors so they do 
not affect the parallel a-c connection. Any number of 
stages may be cascaded in this manner, provided, of 


course, that the transformer will deliver the proper 
voltage to all the rectifiers in parallel. Four stages 
were chosen in this case because four steps of voltage 
are necessary for operation of the iMA-4 tube. The lower 
voltages required for the tube are obtained in the 



Figure 29. Components of Type S-5 power supply. 


22 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


usual wa}^ by a voltage divider across the first section 
of the power supply. A thermionic rectifier (RCA 
1654) is used in this stage in order to supply the 
divider current, but the following stages make use of 
KR31 gas rectifiers, thus eliminating the need for 
filament-supply circuits with a high degree of voltage 
insulation. The current drain at the high voltages is 
very low so that the voltages shown are obtained with 
a total battery current of only 0.4 ampere at 2 volts. 

The chief problems in connection with this supply 
are leakage and corona. These must both be kept to a 
minimum since the internal resistance of the power 
supply is quite high. Leakage can be minimized by use 
of high-quality insulation and protection from humid- 
ity. Corona can be prevented by eliminating all sharp 
edges at the high-voltage connections or by coating 
with a closely adhering insulating material like wax. 

25 INFRARED TELESCOPES 

^ ^ ^ Telescope Performance 

In general, there are two types of application for 
infrared telescopes which require somewhat different 
theoretical and practical considerations when the sen- 
sitivity and resolution of the telescope are to be pre- 
dicted or measured. The first is where the telescope 
is used for signaling or to observe marker lights, and 
the second is where reconnaissance and the viewing of 
extended objects is involved. In the first case, the 
object may be considered a point source, which is 
obviously not true in the second case. 

PoinUSource Sensitivity. When the distance of a 
source is so great that the spot of light cannot be 
resolved by the eye, the eye responds to the total light 
flux entering the pupil rather than to the surface 
brightness of the source. As the distance is increased, 
the visual threshold is reached, and the sensitivity of 
a given telescope is usually expressed in terms of the 
radiation required to produce this threshold response. 
For a given filter, this sensitivity is usually expiessed 
in terms of the mile-candlepower of the uiifiltered 
source. 

The threshold sensitivily of the eye may be taken 
as that of the light of a sixth-magnitude star, or, in 
some instances, as that of a fourth-magnitude star. 
Consequently, a source having a luminous intensity 
of 1 mile- (statute) candle corresponds to a 1. 84- 
magnitude star. A fourth-magnitude star would be 
0.14 mile-candle and a sixth-magnitude star, 0.21 mile- 
candle (0.028 nautical-mile-candle). 


The contractors report^^ derives the following ex- 
pression for the optical efficiency factor for infrared 
telescopes : 


Ep = — m^d\ 

400 K 


where Ep is the optical efficiency factor for the tele- 
scope viewing a filtered infrared source with filter 
factor K ; m is the magnification of the eyepiece 
(ocular) ; is the diameter of the objective; A is the 
optical transmission factor representing light losses 
due to absorption and reflections ; and C is the conver- 
sion of the image tube in lumens emitted per lumen 
incident. 

One other factor must be considered: the ^‘bright- 
ness” of the spot on the screen required to produce 
threshold response in the eye increases with the back- 
ground glow. The background factor B is the number 
of times the “brightness” of the spot must be increased 
as a result of a given background glow, over the 
“brightness” which would be required for threshold 
if the screen were completely black. B varies among 
observers, and it applies only close to absolute thresh- 
old. If a brightness corresponding to I is assumed as 
a threshold, the threshold of the telescope in mile- 
candles — its sensitivity Sp — will be 



Thus, in terms of image-tube cbaracteristics for a 
given source and telescope: 

Sp =k 

where k stands for the constant factors in the equa- 
tion for Ep above. Under actual conditions of use, 
for example in signaling, the “bi-ightness” of the 
spot has to be considerably above threshold in order 
to obtain reliability, and the background factor be- 
comes much less important. A universal sensitivity 
rating probably lies between Sp and a rating which 
omits the background factor. At present, I'ating of 
production image tubes for signaling is primarily 
based on Sp, but a lower limit has been set for C. 

Sensitivity for Extended Objects. For an extended 
image, the magnification of the eyepiece has no effect 
on the brightness of the image seen, as long as the 
exit pupil of the ocular is larger than the pupil diam- 
eter of the eye. The illumination on the photocathodc 
is inversely proportional to the FZ-number of the 
objective, so that in this application the sensitivity 
does not depend upon objective diameter alone. 
Finally, the brightness conversion of the image tube 


INFRARED TELESCOPES 


23 


rather than conversion alone is the determining factor 
for extended objects. 

The ratio of brightness B of the image seen on the 
fluorescent screen to the object brightness Bo is there- 
fore : 

B__ C 

Bo 

where ni is the magnification of the image tube, C 
its conversion for the radiation in question, and F is 
the F/-number of the objective of the telescope. The 
absorption factor, taking into account light losses due 
to absorption and reflection in the optical system, has 
not been included. About 5 per cent loss should be 
allowed for each noncoated surface and 15 per cent 
for each mirror reflection. Nonreflecting films on the 
optical surfaces will greatly reduce losses in a com- 
plicated optical system, but these films must be ad- 
justed to the wavelength of the radiation involved. 
In general, the transmission will be between 25 and 
35 per cent for telescopes of the type employed in 
this work. 

Where a searchlight or spotlight is used to illum- 
inate the object, the requirements for the illuminator 
can be calculated in much the same way as in an 
ordinary lighting problem. If the beam candlepower 
of the source is P, the object is at a distance D (feet) 
and of reflectivity r, its brightness as seen through 
the telescope will be 

P FCr 

The expressions derived above are sufficient to permit 
an estimate of practical lighting requirements. 

Besolving Power. The resolution requirements for a 
reconnaissance telescope are in general more severe 
than for a signaling instrument. Visual acuity of the 
human eye for a well-illuminated object corresponds 
to about 1 minute of arc. As the brightness of the 
object decreases, the visual acuity also decreases and 
corresponds to perhaps 10 minutes when the illumina- 
tion reaches 10“^ to 10“^ foot-candle (assuming good 
contrast and a high reflection coefficient for the bright 
portions of the object). In this range of brightness, 
scotopic or rod vision begins to predominate. Experi- 
ment indicates that this order of brightness is con- 
venient for work with the infrared instruments, in 
that fatigue is not too serious and at the same time 
it is not wasteful of illuminator power. 

If the operating visual acuity is assumed to be 10 
minutes, this means about 1.4 lines per millimeter 
at a 10-inch viewing distance. Therefore, if the image 


tube has a resolution of A == 450 lines, the image can 
be viewed with an eyepiece power as high as 20 before 
instrumental definition limits the performance of the 
telescope. Because these instruments are frequently 
used under conditions which give somewhat better 
eye performance, and also because the definition of 
the objective-image tube combination may not give 
full . 450-line definition, the most practical ocular 
magnification has been found to be 10 to 12. Other 
factors, such as the size of the exit pupil, also are 
taken into consideration. 

The resolving power of the instrument must be 
considered in terms of the type of target being ob- 
served. No increase in range can be obtained by in- 
creasing the illumination if the essential detail of the 
target is below the instrumental limit of resolution. 

For example, to recognize a man against anything 
but the simplest background, the definition in the 
object plane must be 6 inches or better. Assuming 
Type C 2 optics with a 3.5-inch focal length objective, 
the resolution required to give the required 6-inch 
definition in the object plane at 200 feet is 150 lines. 
With a 10-power ocular, this represents about 15 min- 
utes of arc. The object is, therefore, within the limits 
of both visual and instrumental definition. 

With a 2.5-inch focal length objective, such as is 
used in the Type B binocular, the angle at the eye 
becomes 11 minutes and is, therefore, close to the 
limit of visual acuity at these light levels. Where a 
6-power ocular is employed, the definition would not 
be adequate. 

Expected Eanges. The group of reconnaissance in- 
struments to be described, namely. Types C 2 , B, D, 
and K, all employ F/2 objectives. The sensitivity and 
peiformance of these telescopes can be estimated from 
the equations derived above. Assuming a conversion 
C = 1, a magnification m = y 2 , and a transmission 
A = 0.35, the brightness ratio B/Bo = 0.09. In other 
words, the object must be 11 times as bright to pro- 
duce the same sensation through the telescope as 
Avould be obtained without the instrument. If an in- 
frared filter with a filter factor of 10 is used, the ratio 
becomes B/Bq — 0.009. 

It is of interest to determine the ranges that might 
be expected from various illuminators with such tele- 
scopes. First, consider a 30- watt concentrated source 
with a beam candlepower of 50,000. Taking 0.01 foot- 
candle illumination with visible light as the minimum 
for reasonably good ^‘^seeing,” the above estimate in- 
dicates that the filtered source to be used with the 
telescope must be large enough to produce 1.0 foot- 




24 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


candle without the filter. Therefore, the range of the 
source in question is 

D = = 220 feet or 75 yards. 

Employing a 600-watt airplane landing light with 
a beam candlepower of 500,000, the range becomes 
700 feet. Finally, if a searchlight with a beam candle- 



Figure 30. Components of Type C 2 telescope. 


power of 20 million is used, the range should be 4,500 
feet. These ranges are fairly well substantiated by ex- 
perimental observations, but backscattering from dust 
and water particles in the air and absorption tends to 
reduce the larger range values, particularly if the tele- 
scope is close to the searchlight. 

Image brightness is only one of the factors which 
must be taken into account in estimating the working 
range. Object reflectivity and contrast are important. 
In general, under field conditions, poor contrast and 
the like will reduce the working range to 60 or 70 
per cent of the values given above. 


2.5.2 Infrared Telescope Types 

Signaling and Markee Detectors 

The widest and perhaps the most exacting use of 
the electron telescope is for observing infrared signal 
and marker lights. These lights may be used to mark 
important positions such as landing or assembling 
points, ships, airplanes, or airfield locations. The 
source lights may be equipped with shutters or other 
keying mechanism and used for transmitting messages 



Figure 31. Type C 3 telescope — production instrument. 


in code. The lights may be arranged in patterns for 
the transmission of information. 

Type C^. This telescope (Figure 30) was one of the 
first to use the 1P25 in any quantity in the laboratory ; 
about 30 were built, some on Contract OEMsr-440, 



Figure 32. Infrared Schmidt telescope — Type Cf. 


INFRARED TELESCOPES 


25 



the others on separate contracts with the Armed Serv- 
ices who wished to continne tests with them along 
lines initiated by the NDRC project. 

This type operates from two size-D flashlight bat- 
teries, and has a continuous operating life of about 
1 hour per set of batteries. The objective is an jP/ 2, 
3^-inch-focus Cinephor, and the instrument is opti- 
cally focused by rotating the threaded lens mount. The 
eyepiece is a 9X triplet which, in conjunction with the 
hemisphere, gives an overall ocular magnification of 
11. This instrument has an angular field of 18 degrees 
and an absolute threshold sensitivity of Sf = 0.2(y 
for a sixth-magnitude star, and 1.3 for a fourth- 
magnitude star. 

Type C^. As a signaling instrument. Type Cg had 
a somewhat narrow angular field. This could be im- 
proved by a shorter focal length in the objective, but 
would result in an impractically small i<'/-number. 
By the Schmidt system, however, the focal length can 
be decreased and the objective diameter increased. 
Type C3 was eventually evolved, reengineered for 
manufacture, and put into production employing plas- 
tic optics.^ It was used in the fleet in very large num- 
bers before the end of the war. 

The C3 optical system has a focal length of 2.38 
inches and an effective aperture of F/0.9 or better; 
its angular field was about 25 degrees. Assuming an 
optical efficiency factor of 25 per cent, the sensitivity 
is 0.16 for a sixth-magnitude star (nautical) and 0.8 
for a fourth-magnitude star (statute). 

The complete weight of the C3 instrument in pro- 
duction form (Figure 31) was 7 pounds, somewhat 
greater than desirable for a hand-held unit. However, 
since the majority of the instruments were used with 
a signaling searchlight to which they could be attached, 
the weight was not objectionable. 

Some reconnaissance tests were made with Type C3, 
but the lack of depth of focus, the difficulty of adjust- 
ment, and the short focal length made it unsatisfac- 
tory for this purpose in spite of the increased bright- 
ness. 

Type Cf‘ In order to test the value of a really large 
absolute aperture, the Type F (Cp) was evolved. Orig- 
inally, this telescope was built around a large image 
tube with a 3-inch diameter cathode but was later re- 
designed to use the 1P25. The objective was a Schmidt 
system with a 9.4-inch aperture and a 7-inch focal 
length. Figure 32 is a schematic drawing of this tele- 
scope and Figure 33 shows it mounted for use. A vari- 
ety of eye lenses permit magnifications from 4 to 10. 

‘’See STR Division 16, Volume 1, Section 8.5. 


Figure 33. Type Cp mounted for use in signaling or 
reconnaissance. 

Maximum sensitivity is obtained with the highest 
power eyepiece, and for signaling the resultant loss of 
field is not too serious considering the gain in sensi- 
tivity. After preliminary tests with the laboratory 
instrument, the Navy negotiated the procurement of 
50 instruments of this type. 


26 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


Even higher signaling sensitivity is possible with 
the high-voltage MA-4 tube, and an instrument of 
this type, using the 7-inch focal length Schmidt sys- 
tem, was built, but only preliminary tests were made 
before termination of the contract. 

Later Portable Telescopes. Two very portable labo- 
ratory instruments. Types L and P, useful both for 
signaling and reconnaissance, were built. The Type L 
followed the general style of the Type C 2 but operated 



Figure 34. Type T telescope, with single-voltage image 
tube. 


from a single flashlight-size storage battery Type BB- 
200/u. The power-supply components were also con- 
siderably smaller than in the C 2 . Type P used a power 
supply identical with Type L, but was assembled in 
a different form for convenience of handling. 

The Type T telescope illustrated in Figure 34 was 
designed around the single-voltage tube. Instead of the 
usual vibrator, an impulse-power supply controlled by 
a balance wheel was employed. This power supply, 
operated from a single size-D flashlight battery, gives, 
instead of the 1- to 2-hour running life of the conven- 
tional instrument, more than 25 hours continuous 
operation. 

Following along the lines of the Type T, a very 
compact version of the impulse-power supply w'as built 
which, together with the small single-voltage image 
tube (Section 2.3.1), was used to convert a Type 
A metascope (Chapter 3) into an electron telescope. 
This change was carried out in such a way that the 
Schmidt objective and eyepiece of the Type A could he 
used without modification, and that almost no changes 
were required on the housing. Only one of these instru- 
ments was built, and is shown in Figure 35. 

The image obtained with the instrument was quite 
good and its performance seemed satisfactory. Sensi- 
tivity measurements were made on it at the Naval 
Research Laboratory, Anacostia, where its threshold 
was found to be 1.5 mile-candles. However, it is doubt- 


ful that it was working properly when these measure- 
ments were made since tube performance and optical 
considerations indicate that the threshold should be 
better than this by a factor of at least two or three. 

Rfx’Onnaissance Telescopes 

The reconnaissance possibilities of the infrared tele- 
scope combined with a searchlight were recognized 
early in the work and the development of this aspect 
was carried on throughout the contract. Three general 
procedures were followed: first the use of a large 
searchlight and a relatively long focal length telescope, 
second, the use of large searchlights at some distance 
from the oljjects being viewed with hand-held instru- 
ments carried by observei's moving up close to the ob- 
ject, and finally the employment of small hand-held 
telescopes and a portable hand spotlight. 

Type As already mentioned, the F /-number 
of this instrument is about 0.9. Therefore, the bright- 
ness ratio for whole light is B/Bo = 0.3, assuming an 
optical transmission of 25 per cent. With a filter 
factor of 10, the ratio becomes 0.03. The field of view 
is about 9 degrees; angular definition is approximately 
2.2 minutes, which is sufficient to distinguish a man 
at about 800 feet with lighting corresponding to 0.01 
foot-candle. 

For the first type of procedure mentioned above, 
the Type Cf telescope was mounted close to a pair of 
3-kilowatt searchlights and a special generator pro- 



Figure 35. Components of converted Type A metascope. 


vided by General Electric Company [GE]. The com- 
plete unit is shown in Figure 17, Chapter 5, of this 
volume. This application was shown at the demonstra- 
tion at Solomon’s Island on August 25, 1942. The 
1 ‘anges obtained then were rather small, but improved 
techniques eventually made it possible to see a shore- 
line and buildings along a coast up to a distance of a 
mile, and to detect (but not to identify) trucks, tanks, 
and other large vehicles at 700 yards. When the instru- 




INFRARED TELESCOPES 


27 



nient and searchlights are carried in a large boat and 
used to pick u]) prominent objects along a coast, portable 
instruments carried in small boats using the illumina- 
tion from the searchlights for close-up inspection are 
of considerable aid. The combination of marker and 
signal lights to guide the overall operation with these 



Figure 37. Components of Type D telescope. 


two forms of reconnaissance should make a very pow- 
erful team for certain types of amphibious operations 
against an opponent not equipped with counterin- 
struments. 

Type Cg. This instrument, which has already been 
described for its use in signaling, was also employed 
for reconnaissance, although the types described be- 
low superseded it. 

Types B and D. Both the Types B and D instru- 
ments are based on the telescope shown in Figures 36 
and 37 (Type D), and Type B is shown in Figure 
38. Type D consists of a single telescope barrel ; Type 
B is a pair of these mounted to form a binocular. The 
image-tube barrel is of 30-mil mu-metal which pro- 
vides magnetic shielding against external fields. The 
objective is an F/2 plastic lens of 2.5 inches focus. 
A field-corrector lens matches the image to the curva- 
ture of the photocathode. 


The cable from the instrument goes to a small 
battery-operated power supply held in a plastic case. 
One or two focus controls are provided, depending on 
whether for monocular or binocular. This power sup- 
ply, S -1 and S-3, has already been discussed. 

Type D proved to be a very useful instrument. Fre- 
quently, it was used by an observer accompanying the 
driver in night-driving operations. The observer was 
able to walk about (including in front of the vehicle) 
to inspect suspicious objects. In addition to general 
reconnaissance. Type D was used as a signal receiver. 
An instrument very similar to Type D, the C 4 , with 
an improved power supply (S-4), was ordered in 
quantity by the Navy, but deliveries were only start- 
ing when the war ended. 

Type B also proved to be very useful, but it was not 
put into production. The hinge between the barrels 
allows interpupillary adjustment, and eccentric loca- 



Figure 38. Type B binocular unit. 


tion of the eyepieces with reference to the axis of rota- 
tion of the mount permits registering and fusing the 
two images in both vertical and horizontal directions. 
Since these instruments were designed for night 
driving, the majority of them were provided with 2.5- 
inch objectives and 8 X oculars, giving an overall 
magnification of unity. Some, however, had 3. 5 -inch 





28 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


objectives and IX eyepieces, giving a magnification 
of 2.1. The angular field of the former was about 
25 degrees, and of the latter about 18 degrees. 

Type K — Sriooperscope and Sniperscope. The most 
effective application of the small reconnaissance tele- 
scopes was in the Type K instruments, the snooper- 
scope and sniperscope. These are illustrated in Figures 
39 and 40. Both of these instruments were based on 
the Type D telescopes, but used 3.5-inch objectives. 

In the case of the snooperscope, the telescope and 
30-watt sealed-beam lamp (see Chapter 5) were 



Figure 39. The snooperscope. 


mounted on a light handle, the unit weighing about 
6 pounds. The power supply and batteries for the 
lamp were carried in a knapsack which weighed 13 
pounds. The batteries were sufficient for 31/2 to 4 
hours’ continuous operation. 

The same telescope, lamp, and power supply were 
used for the sniperscope. The lamp and telescope were 
mounted on a carbine, as shown in the photographs, 
and added about 5 pounds to the weight of the gun. 
An opaque chevron recticle was applied to the field- 
corrector lens on the side in contact with the image 
tube to cast a shadow on the photocathode. It was 
accurately aligned with the optics of the telescope 
and served as gunsight. With this unit, it is possible 
to hit a target the size of a man against an average 
background at a distance of 75 yards, and at a greater 
distance against special backgrounds. The Army 
placed procurement orders for a fairly large number 
of these two types of instruments, and a quantity 
(2,000 to 3,000) were used in the latter stages of the 
Pacific campaign. 

Experience with the sniperscopes showed that there 
would be an advantage in mounting the telescope on 
heavier rifles, or even on machine guns. The use of 
larger separate sources may be advisable, and a more 


sensitive telescope using a high-voltage image tube 
would be desirable. 

Night-Driving Instruments 

Eaidy Instruments. The first type of instrument 
used to any extent for night driving after the Aber- 
deen test (Section 2.1) employed an image tube with 
a 3-inch photocathode and a magnification of ^ 2 .- It 
is interesting to note that the German night-driving 
equipment which they were about to put into opera- 
tion when the war ended was very similar to this unit. 
With the reduction in size of the image tube, first to 
the 6-inch size and then to the 4y2-inch 1P25, it was 
possible to reduce the size of the telescope and to 
develop stereoscopic binoculars. 

For tank driving, a ^^protectoscope” was designed 
to fit into the viewing hatch, with the infrared tele- 
scope occupying about one third of the visual chan- 
nel. The tests made with this unit seemed to indicate 
that it was a fairly practical solution to night opera- 
tion and fighting with tanks with closed armor. An- 
other instrument — a helmet unit — was designed to be 
used as a tank periscope, but was too heavy to be worn 
without undue fatigue. Tests at Aberdeen on August 
12, 1942, demonstrated the complete feasibility of 
driving tanks in full darkness, and of operating them 
by infrared when closed for combat. 

When periscopic binoculars were designed with 6- 
inch image tubes, they gave better results than any 



Figure 40. The sniperscope. 


of the previous instruments, but they were too com- 
plicated in design and very difficult to keep in cor- 
rect adjustment. 

Type B. Mounted in jeeps, ducks, tanks, Type B 
binoculars had all the advantages of the periscopic 
instruments yet were simple enough to readily remain 
in adjustment. With these instruments, ordinary head- 
lights with infrared filters permitted an average driv- 
ing speed of 8 to 10 miles an hour; with 150-watt 
lights, the speed could reach 30 miles per hour; and 
if 500 to 1,000 watts were used, it was possible to 


h || I 


INFRARED TELESCOPES 


29 


drive fully as well as with ordinary visible night- 
driving lights. The installation on a jeep is shown in 
Figure 41, and in Chapter 5, Figures 11 and 12 show 
the installations on an aniphihian and a tank. 

Infrared illuminators are an important part of 
night-driving development. After the initial experi- 
ments, this problem was handled l)y GE under Con- 
tract OEMsr-4213, because of their experience with 
road lighting and headlights, and the sources used 
are described in Chapter 5 of this volume. 

Type Z — TIehnet I fist rumen fs. The Type B binoc- 
ulars overcame most of the difficulties of the earlier 
night-driving systems. However, when used on rough 
roads at high speeds the motion of the eye relative to 
the ocular of the instrument became somewhat objec- 
tionable, and further Avork was done in trying to 
stabilize the binocular mounting. 

However, tbe final step in the night-driving deA’el- 
opment Avas the helmet unit shoAAUi at the right in 
Figure 42. This unit Avas AAajrked ont under Contract 
OEMsr-10T5 AAuth the Johnson Foundation of the 


UniA^ersity of Pennsylvania, using telescope practice 
developed by RCA and the large amount of experi- 
ence in general helmet design on the part of the staff 
of the Johnson Foundation. The general arrange- 
ment of the components can be seen at the left of 
the figure. With the exception of the ocular magnifiers, 
plastic optics Avere used throughout. The objectives 
had a 2 y 2 -inch focal length and a numerical aperture 



Figure 41. Jeep equipped for night driving Avith infrared. 



PRISM 

RED FILTER 

SCOTCH TAPE 
SPACER 

MU - METAL 
SHIELD a CAP 

SPACER a 
WASHERS 

IMAGE TUBE 


MAGNETIC 

ALIGNMENT 

RINGS 

SOCKET- 


LUCITE 
ADJUSTING RINGS 


OCULAR - 


PRISM" 


OBJECTIVE 


VOLTAGE DIVIDER 


FOCUSING POTENTIOMETER 


Figure 42. Telescope unit for Type Z binocular. 


30 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


F/2. Tlie overall magnification was unity. The gen- 
eral design of the telescope followed the practice al- 
ready described, except for detail. A somewhat dif- 
ferent method of obtaining register of the two images 





Figure 43. Power supply (S-4) for helmet binocular. 

was employed than for the in-line binoculars. Two 
steel rings which were magnetized so as to produce 
magnetic fields across a diameter were placed around 
the telescope barrel just below the mu-metal shield. 
By rotating these two rings independently, the elec- 
tron image in the ir25’s could be moved into any 
desired position. Type S-4 power supplies shown in 
Figure 43 were employed. 

The pair of telescopes and head harness weighs 2 V 2 
to 3 pounds while the j^otver supply weighs 2 V 2 pounds. 
When mounted on a steel helmet as shown in Fhgure 
44, with the power supply acting as a counterbalance 
to the binocular, the additional five pounds on the 
driver’s head was quite unobjectionable. 

Twenty of the helmet binoculars were built under 
Contract OEMsr-440, twelve for test on night driving 
and eight for night flying. Just before the end of the 
war, as a result of tests by the Engineer Board at 
Fort Belvoir, the Army was negotiating the procure- 
ment of 100 instruments as a preliminary to placing- 
production orders. 

Instruments for Airborne Operations 

Tests showed that the near infrared telescopes were 
not satisfactory for detecting planes by their own ra- 
diation, except where the plane had a long length of 
exposed exhaust pipe or other hot surface. Conse- 
quently this application was not pursued further, but 
there are several other applications of infrared tele- 
scopes to airborne operations. 


Glider Towing and Landing. This problem requires 
an infrared telescope through which lights on the tow- 
plane may be observed. The telescope should, in gen- 
eral, be fixed to the glider and have in it a visible ref- 
erence index with respect to which the marker lights 
may be aligned. The tow-plane should carry wing tip 
and tail lights. 

Two large telescopes similar to those made initially 
for driving were built and mounted on gliders. These 
instruments employed a projection reticle which gave 
a frame of reference fixed with respect to the glider. 
Like the driving instruments, these telescopes were 
nonstereoscopic, hut allowed binocular vision, and were 
arranged so that the pilot did not have to hold his head 
close to the instrument. These instruments were used 
both for flying and landing, and were in general fairly 
satisfactory (Wright Field, Dayton, Ohio, May 5 to 
21, 1943).« 

Inasmuch as the units just described were quite 
large, and it was found that the pilot preferred to 
view the ocular from the closest possible distance, it 
was decided that the Type B^ binocular might form 
the basis of a more satisfactory instrument. Conse- 


p V 



\ 


. - 

Figure 44. Combat helmet binocular (Type Z). 

quently, the unit shown in Figure 45 was constructed, 
consisting of binoculars which clipped into a cradle 
which carried a headrest and the projection reticle. 
The binoculars were used in this cradle for towing or 
landing operations, but could be removed and used as 
a hand instrument for locating landing fields or mark- 
ing light, checking identification, etc. This instrument 
was turned over to Wright Field for further tests. 


INFRARED TELESCOPES 


31 



Figure 45. Binoculars mounted for use in glider 
towing and landing. 


Identification — Type It. One of the i^robleins of night 
flying is the identification of friendly planes to avoid 
their being accidentally shot down. A possible solution 
is to equip planes with infrared identification liglits 
and telescoj^es. The telescopes for this purpose must 
be small and placed so as not to interfere with the 
planers operation. They also must be easily viewed, 
both from a distance and near by. 

The barrel and objective of the instrument devel- 
oped for this purpose is the same as that used in the 
Type 1). The eyepiece is, however, special, being a 4- 
power magnifier with a diameter of about 2 inches, 
which in combination with the hemisphere of the im- 
age tube forms a 6-power ocular. The latter is adjusted 
so that the virtual image is at infinity so that the im- 
age may be viewed from a distance up to about 15 
to 20 inches without change of sensitivity of the in- 
strument. The eye lens permits monocular vision only. 

A second eye lens made from a Mark VIII gunsight 
ocular was also investigated. This system was pro- 
posed by the staff of Naval Research Laboratory. When 
adjusted so that the virtual image is 10 inches in 
front of the ocular, it i^ermits binocular vision. It 
suffers, however, from two disadvantages. The sensi- 
tivity of the telescope decreases by a factor of 80 when 
the observer moves from close to the instrument to a 
point 20 inches back of the ocular. Furthermore, the 
image is very seriously barrel-distorted. For these 
reasons, the first described eye lens is considered more 
satisfactory. 

B-29 Tail, ng lit Telescope. A somewhat more com- 


plicated identification telescope is shown in Figure 
46. This was designed to be used on the computing 
gunsight in the tail of a B-29. The eye lens, telescope 
barrel, and objective are similar to the previously de- 
scribed instrument. Ahead of the objective, there is 
a pcriscopic section which transfers the observeFs eye- 
point to the center line of the sight. This is necessary 
since one of the requirements of the telescope was that 
it have the same angle of vieAv through the rear win- 
dow as the sight itself. A projection reticle was ar- 
ranged to project index marks on the cathode. These 
marks indicate the position of the image of lights on 
the wing tips of a B-29 a specified distance behind the 
observing plane. 

This telescope was sent to Wright Field for test, 
where only a very small amount of work was done 
with it. 

Night-Landing Telescopes. The landing of aircraft 
on blacked-out airfields, or carrier decks, can be ac- 
complished by means of infrared telescopes. Prelimi- 
nary tests indicated that a fixed instrument, such as 



Figure 46. Type R telescope on B-29 tail gunsight. 


the glider-type telescope, would not, in general, be suit- 
able. Further tests with a lightweight periscopic 
monocular (Type H) mounted on a helmet showed 
that similarly mounted binoculars, if correctly de- 
signed, could be used for night landings. 

Although the head harness required for the flying 
instrument is very different from that for driving. 




32 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


the folded Type Z telescopes can be the same, and the 
development of the units undertaken by the Johnson 
Foundation, as has already been mentioned, was for 
both purposes. 

The telescopes themselves have already been de- 
scribed in connection with the driving application. The 
head harness (Figure 47) is arranged so that the tele- 
scopes can be removed or replaced readily with one 
hand, while the harness is in place on the head. When 
the telescopes are attached, they can be pulled down 
over the eyes in the viewing position, or tilted back 
out of the way. When in viewing position, the harness 
is tight around the head to hold the instruments ac- 



Figure 47. Type Z helmet binocular for night flying. 


curately in place, but the tension is released on tilting 
them back. The binoculars are so arranged that they 
can be worn over standard flying goggles. The ocular 
ends are small enough so that the user can readily see 
his instrument panel under them. When the binocu- 
lars are removed, the weight of the harness is negli- 
gible and it can be worn for long periods without dis- 
comfort. The complete unit in operating position does 
not produce serious discomfort and can be worn with- 
out difficulty for periods many times longer than 
would be required in practice. The harness weighs un- 
der 1 pound and the binocular about 2 pounds. 

A great many landing tests were made without ac- 
cident or mishap. These included work with a civilian 
plane by members of the Johnson Foundation staff 
and with military aircraft by both Army and Navy. 
It was concluded that the instrument provided a fairly 


satisfactory means of night landing where a complete 
blackout is required. It was found that it was per- 
fectly possible to make carrier landings with these in- 
struments where the carrier deck was marked with in- 
frared runway lights, and the signal man used in- 
frared luminous wands. Further details on the solu- 
tion of this problem, including a description of the 
lighting, will be found in reports by and the 

Johnson Foundation.^® 

Paratroop Assembly Operations. Here the infrared 
telescope is used to pick out marker lights placed by 
a scouting group to identify the landing or assembly 
location. Since the marker light must be as small as 
possible (see Chapter 5), Type Cg telescope was se- 
lected as having the high sensitivity required by the 
pilot of the plane in discerning the marker. This type 
of operation was tested and found to be very feasible. 

Once the area is located, the men who make the 
jump may carry small, lightweight telescopes and in 
frared flashlights. The instruments may be either 
metascopes or small infrared telescopes with which 
they can locate the assembly beacon. This type of 
operation was tested and also found very feasible. 

It is suggested that sniperscopes and snooperscopes 
carried by a number of the men would be of value. 
Because of the danger of any of these instruments 
falling into enemy hands, they should be made self- 
destroying after a certain interval of time. 

Special Telescopes 

In order to accommodate the special tubes, type 
MA-4 and the high-voltage low-magnification tube, 
special telescopes were designed. 

Type IF. A telescope using a Type MA-4 image tube 
retained the general shape of Type Cg, but was some- 
what larger in order to accommodate the high-voltage 
power supply. The instrument is made in such a way 
that alternative objectives could be used. One objec- 
tive is a 3.5-inch focal length F/2 Cinephor; the other, 
a projection lens with a 4.75-inch focal length and a 
relative aperture F/1.4. 

The internal arrangement of the instrument can 
be seen in Figure 48. The power supply delivers a 
total of 16 kilovolts to the image tube. Because of the 
high voltage employed, great care has to taken in in- 
sulating various of the components. A 2-volt storage 
battery (flashlight battery size) operates the instru- 
ment, with a continuous running life of IV 2 to 2 
hours. Provision was made for operating the unit from 
an external 2-volt battery if desired. 


SUMMARY 


33 


The performance of the instrument was excellent, 
its conversion being about 8 times that of an instru- 
ment using ir25, so it gives a much brighter picture. 
Its definition is somewhat better than that of the con- 



Figure 48. Components of Type W telescope. 

ventional telescope, and, because of the sulfide screen 
used in the MA-4, it has no observable time lag. 

A telescope based on the Schmidt system used for 
the Type Cp was also built incorporating the MA-4. 
The optics for this telescope were obtained from the 
Bureau of Ships and tests were made by it with fairly 
satisfactory results. However, the optics of this instru- 
ment could have been improved so as to take full ad- 
vantage of the capabilities of the Type MA-4 image 
tube. 

High-Voltage, Low-Magnification Telescope. The 
high-voltage, low-magnification tube is by far the most 
sensitive image tube developed during the contract. It 
offers a possible means of being able to see under con- 
ditions of illumination below that required by the un- 
aided eye. Tests made with an experimental telescope 
employing this tube proved that it was possible to see 
objects illuminated by whole light from an incandes- 
cent source when the light level was too low to see 
them with the naked eye. 

As was pointed out in the discussion of low-magni- 
fication tubes, there are limitations to the increase in 
brightness that can be obtained in this way. These lim- 
itations are characteristic of the telescope rather than 
the tube. The brightness of the observed image only 
continues to increase as long as the exit pupil of the 
ocular exceeds the diameter of the pupil of the ob- 
server’s eye. The diameter of the pupil of the dark- 
adapted eye is about 6 millimeters. Therefore, the prac- 
tical limit to the magnification of the ocular if a 
compound microscope is employed is about 100, while 
the limit for a simple magnifier is about 30. 

It is easily showiT^ that B/Bo may reach 1,500. Ac- 


tually, it will be some time before any such gain can 
be realized, because there are many practical difficul- 
ties both of telescope and image tube which must be 
overcome. 

As has already been mentioned, the most practical 
high-voltage, low-magnification image tube developed 
under the contract had a magnification % to Vg. 
The tube operated with an overall voltage of 20 kilo- 
volts and had a conversion of about 3. It was used in 
a telescope with Schmidt objective of 7-inch focal 
length and an /-number, F/0.9. Various oculars were 
used with this instrument ranging from 10 X to 15 X. 
Because of the high brightness conversion of the tube, 
it was necessary to cool the photocathode. This was 
accomplished by means of a small liquid-air container 
arranged to blow cold air against the photocathode. 
Precautions had to be taken to prevent the condensa- 
tion of moisture on the optical parts. With the 15 X 
eyepiece, the magnification is 1.3 and the brightness 
ratio, 15. All of this gain was not realizable because 
of loss of contrast and the like, but there was a defi- 
nite gain over direct vision when incandescent lights 
were used. This line of development was considered 
one of the most promising and should be continued, 
both as applied to visible light and infrared radiation. 

2 6 SUMMARY 

Early in World War II, it was decided to exploit 
the possibilities of the infrared-electron telescope for 
nocturnal vision. Section 16.5 undertook the develop- 
ment of the necessary image tubes and instruments 
and investigated various applications. Before the close 
of the war several thousands of these telescopes had 
been put in service by the Navy, primarily for night- 
time communications, and a similar number in the 
form of sniperscopes and snooperscopes were in use by 
the Army in the Pacific campaign. Other instruments 
in smaller number had seen service for a variety of 
purposes. 

The image tube is the essential element in the in- 
frared-electron telescope and serves to convert an in- 
visible infrared image into a visible image. The tube 
consists of a semitransparent photocathode sensitive 
to the radiation in question, an electrostatic electron 
lens system, and fluorescent screen. An infrared image 
on the photocathode causes electrons to be released in 
conformity with the image. These electrons are ac- 
celerated and focused, by the electron optical system, 

® Taken from bibliographical reference 1, with changes in 
style only. 




34 


INFRARED IMAGE TUBES AND ELECTRON TELESCOPES 


onto the fluorescent screen where <a visible reproduction 
of the original image is formed. 

Work on the image tube was started before the war. 
Under the NDRC contracts, the tube was developed to 
the point where in 1942 it could be put into produc- 
tion as the 1P25. The investigation then continued 
along the lines of improving the 1P25, developing a 
replacement single-voltage tube, working out a small 
high-voltage tube and studying special tubes. A single- 
voltage tube was designed so that it could be used as 
a replacement for the 1P25 in the then existing in- 
strument and in new instruments which were much 
smaller and longer lived because of simpler, lower 
power supplies. A very satisfactory multiple-anode, 
high-voltage image tube was developed which was at 
the point of going into production at the close of 
World War II. The study of special tubes included 
the following: (1) work on a high-voltage, low-mag- 
nification infrared image tube which, when used in a 


Basically, the infrared telescope consists of an ob- 
jective which images the scene being viewed onto the 
photocathode of the image tube, and an ocular ar- 
rangement for viewing the reproduced image. 

An electrical power supply furnishing 4,000 to 
6,000 volts is required to actuate the image tube. The 
obtaining of the required high voltage from a small 
lightweight unit presented some rather special design 
problems. A combination of vibrator, transformer, and 
rectifier was developed which gave excellent results. 
Two new rectifiers were developed, one of which 
(RCA 16o4) is finding wide usage in other applica- 
tions. 

A large number of different types of telescopes were 
designed and built to meet the needs of various ap- 
plications, such as signaling, night driving, night fly- 
ing and landing of airplanes and gliders, airplane 
identification, reconnaissance, and gun aiming. Some 
of the instruments developed for these operations aie : 


Type 

Optics 

Power Supply 

Application 

C 2 

F/4 refractive 

Self-contained 

Signaling and general purpose 

Ca 

2.4 Schmidt 

Self-contained 

Signaling 

D (forerunner of C 4 ) 

F /2 refractive 

Separate 

General purpose 

B (binoculars) 


Separate 

Reconnaissance and general purpose 

R 

Ki (sniperscope) 

Special ocular 

Separate 

Plane- to-plane identification 

Gun aiming 



K 2 (snooperscope) 

Z (helmet-mounted binoculars) 

Cf 

Reconnaissance 



Night driving and flying 

Signaling and reconnaissance 

7-inch Schmidt 

Separate 

L 

Refractive 

Small self-contained 

Signaling and reconnaissance 

T 

Refractive 

Impulse 

Signaling 

CW 

Refractive 

High voltage 

General purpose 

CV (low magnification) 

7-inch Schmidt 

High voltage 

General purpose 


suitable telescope, gives promise of exceeding the eye 
in sensitivity even for ordinary visible light; (2) an 
investigation of image tubes for the intermediate and 
far infrared portions of the spectrum; (3) image 
tubes having special electron optics, phosphors, or pho- 
tocathodes. 

In addition to the development of image tubes 
themselves, detailed studies were made of the various 
components going to make up these tubes, including 
photocathodes, electron lenses, and fluorescent screens. 


Many other instruments were built and used in the 
course of the tests. 

Continued research in this field should include fur- 
ther work on high-voltage tubes and instruments for 
near infrared application, continued investigation of 
high-voltage, low-magnification image tubes with a 
view toward achieving an instrument which will per- 
mit vision at visible light levels below which the un- 
aided eye is operative, and the development of a far 
infrared-imaging device. 




Chapter 3 

METASCOPES 

By Mary Banning^ 


31 INTRODUCTION 

A SERIES of image-forming infrared receivers which 
convert infrared to visible light have been de- 
veloped at the Institute of Optics, University of Roch- 
ester, under NDRC Contracts OEMsr-510 and 
OEMsr-1219. With these instrnments, a source of in- 
frared radiation, or objects illnminated by it (see 
Chapter 1), can be seen by an observer in much the 
same manner as objects emitting or illuminated by 
visible light can be seen with an ordinary telescope. 
These infrared receivers depend for their performance 
upon special jdiosphors converting infrared to visible 
light and upon specially efficient optical systems. The 
phosphors will be described in Chapter 4. At the re- 
quest of the Navy Bureau of Ships for a descriptive, 
yet concealing term for this class of infrared receivers, 
the name nietascope has been api3lied. 

A metascope consists essentially of a high-aperture 
optical system which forms an image of distant objects 
upon the surface of an infrared-sensitive phosphor. 
This image covers a useful field of 25 to 45 degrees, 
depending upon the particular style of instrument. 
A second optical system is provided for viewing the 
phosphor surface, which emits visible light on stim- 
tdation by the incoming infrared. Since the operation 
of the metascope depends on the phosphor’s emitting 
visible light of a shorter wavelength than the infrared, 
it is necessary to provide exti’a energy by exciting or 
charging the phosphor prior to its use. This excitation 
may be accomplished by visible or ultraviolet light or 
by radium, all three methods having been applied suc- 
cessfully. In each case, the exciting means are so 
provided that the metascope is entirely self-contained. 
Several forms of these instruments were put into pro- 
duction by the Army and Navy, for operational use 
in both the European and Pacific theaters. They were 
used by the Navy for signaling and identification and 
by the Army for signaling, especially applied to the 
assembly of airborne troops in a drop zone at night. 
The weights of the production instruments range 
from about 4 pounds for the heaviest instrument de- 
veloped to about 3 ounces for the lightest. 

•Institute of Optics, University of Rochester, Rochester, 
New York. 


Optical Design. All metascopes have the same fun- 
damental optical design, and make use of the large rela- 
tive aperture of the Kellner -Schmidt [K-S] system. 
In essence, the K-S system is as shown in Figure 1. 
Here 4/ is a spherical mirror, C is an aspheric plate 
placed at the center of curvature of the mirror to cor- 
I’ect for spherical aberration, and F is the focal sur- 
face. In the metascope, the phosphor is coated on or 
otherwise formed to occupy this focal surface. The 



Figure |1. Sectional view of Kellner-Schmidt system: 
Af, spherical mirror; F, focal surface; C, corrector plate; 
D, aperture; /, focal length. 


most important characteristics of the K-S system are 
its great simplicity and its wide field of definition, 
combined with large aperture-ratio. 

Infrared radiation entering through the corrector 
plate is reflected by the mirror to form an inverted 
real image on the phosphor. The brightness of this 
image is directly proportional to the flux gathered. 
In the case of a point source, it is proportional to the 
square of the aperture D. If the source is an extended 
area, it is also inversely proportional to the square of 
the focal length /, that is, it also depends on the size 
of the image formed. For maximum image brightness 
of an extended source, therefore, the aperture-ratio 
should be as great as possible; the brightness of an 
extended image on the phosphor is thus determined 
only by the speed of the primary system, not by the 
size of the aperture. The immersion of this piimary 
system in a medium of refractive index n, higher than 
that of air, will increase the illumination on the phos- 
phor of an extended image but will not affect the in- 
tensity of a point image. 




35 


36 


METASCOPES 


The intensity of the visible image emitted by the 
phosphor is usually a linear function of the stimulat- 
ing radiation, and is determined by the rules given 
above. The intensity of a point image as seen by the 
eye, on the other hand, will depend on the amount of 


system and the resolving power of the phosphor itself. 
It is thus dependent on the size of the system. 

Methods of viewing the visible image differ in the 
various metascope models and will be described in the 
next section. 



Figure 2. Two-inch ultraviolet telescope, assembly drawing. 


flux received by the eye from the phosphor, or on the 
solid angle subtended by the pupil of the eye at the 
phosphor. 

In an air system, the solid angle utilized increases 
inversely with the square of the focal length f of 
the viewing system, since if it is properly designed 
the limiting aperture is the pupil of the eye, and thus 
the apparent intensity of a point source varies as 1//'“. 
An extended source, however, will not change its ap- 
parent brightness with a change in focal length, since 
the gain in flux is counteracted by the magnification 
of the image. If the viewing system is immersed in 
a medium of high index, there will be no change in 
the apparent intensity of a point source, but an ex- 
tended source will be magnified without a compen- 
sating flux gain and will appear less bright. 

Considering both primary and viewing systems to- 
gether, with unit magnification (/ == /'), both the 
apparent brightness of an extended source and the 
apparent intensity of a point source will vary with 
(D/fY. In neither case is the size of the system im- 
portant, but only the relative aperture. Immersion in 
a high index will not affect the point source, and 
neither will it affect the apparent brightness of an 
extended source since the gain in the primary system 
is compensated by the loss in the viewing system. 

The resolving power of such a K-S phosphor sys- 
tem is determined by the focal length of the primary 


32 development of INSTRUMENTS 
3.2.1 Type A 

Early in 1941, an ultraviolet telescope provided 
with a high-sensitivity ultraviolet phosphor was de- 
signed using a K-S optical system. Figure 2 shows a 
scale drawing, and Figure 3 a photograph of the in- 
strument. 

In this prototype, the visible image emitted by the 
phosphor is viewed through a central hole in the mir- 
ror by means of a lens-erecting system and an eye- 



Figure 3. Two-inch ultraviolet telescope. 




DEVELOPMENT OF INSTRUMENTS 


37 


piece. Lens erection was chosen to give exact unity 
magnification by final adjustment of the lens separa- 
tions. The diameter of the K-S housing chamber was 
made small enough to permit using the instrument 
as a monocular and at the same time to allow unob- 
structed vision straight ahead with the unaided eye. 
Any object seen through the telescope then appears 
the same size and in the same position as when seen 
with the unaided eye, independent of the motion of 
the telescope. This permits use in a fast-moving ve- 
hicle and also allows an observer to locate the source 
of invisible radiation in its proper place against a 
dimly visible landscape. 

This ultraviolet instrument has an aperture of ap- 
proximately 2 inches and works at a speed of //0.55. 
The K-S plate corrects for all rays striking the phos- 
phor up to an angle of 08 degrees. The corrector plate 
was ground and polished by hand methods, from Corn- 
ing glass No. 791. 

An infrared phosphor coated on the focal surface 
was the only change necessary to convert the instru- 
ment for infrared use, although a redesign of the cor- 
rector plate for the longer wavelength is necessary for 
optimum image quality. 

Since the infrared phosphors, unlike those for the 
ultraviolet, require excitation prior to use, it was de- 
sirable to provide some means for excitation inside 
the instrument. An outside attachment would have 
been too bulky. Fortunately, previous work with the 
ultraviolet telescope had shown that there is a location 
between the corrector plate and mirror at which a 
point source will give approximately uniform illu- 
mination over the focal surface, and mechanical pro- 
vision for this had been made in the prototype. A 
small light, provided with suitable filters, can thus be 
placed in the proper position for exciting the phos- 
phor. This method of excitation is satisfactory, except 
that when in use the viewing of an infrared source 
must be interrupted during excitation. 

In May 1942, a request came from the Bureau of 
Ships [BuShips] through NDRC for a compact and 
lightweight infrared system capable of receiving sig- 
nals over a range of 5 to 10 miles and covering as 
large an angular field as possible. It was decided to 
design a 2-inch telescope like that described above, but 
with a built-in exciting system that would permit 
continuous observation. The final design used the 
same optics as the ultraviolet telescope, with the 
dimensions slightly altered. Figures 4A and 4B show 
this first metascope. Type A. It has a clear aperture 
of 2^/4 inches, a field of 36 degrees and a speed of 


//0.55 ; the image is erect and the magnification, unity. 

The phosphor used in Type A is Standard VI, 
described later in this chapter. Standard VI emits 
orange light, and is stimulated by radiation of wave- 
leiigths from 0.75 to 1.3 microns with a well-defined 
maximum at 1.025 microns. It is excited with a tung- 
sten source filtered by Corning light blue-green glass 



Figure 4. A. Front view of Type A metascope. 
B. Side view. 


No. 428 plus Corning medium aklo infrared absorb- 
ing glass. Ten to 30 seconds’ exposure of the phosphor 
to this source is sufficient for charging. Immediately 
thereafter the phosphor shows a bright afterglow 
which soon dies down. The greatest phosphor sensi- 
tivity is 5 to 10 minutes after charging, and recharg- 
ing should be done every few hours. 

Provision for Continuous Viewing. Two extension 
pockets were added on either side of the chamber car- 
rying the K-S system, and two independent focal sur- 
faces coated with phosphor were provided, mounted 
on the arms of a rigid, two-tined fork. The fork is 
pivoted in such a way that throwing a lever on the 
front of the instrument brings one phosphor into the 
focal position and puts the other in a side pocket. 
Each pocket is equipped with a small lamp, suitably 
filtered. On the fork carrying the phosphors is a switch 
which automatically connects only the lamp in the 


38 


METASCOPES 


side pocket occupied by a phosphor. The battery cir- 
cuit can then be closed by a small push button in the 
rear of the case. 

A semicircular guard around each focal surface al- 
lows the operator to excite a phosphor in a side pocket 
without visible radiation escaping from the instru- 
ment. He can then use the metascope with one phos- 
phor surface while exciting the other and without 
disclosing himself to an enemyy or, more important, 
without interfering with his view. Thus, continuous 
viewing is provided. The power supply for the lamps 
consists of two ‘^pejilite’^ dry cells mounted in the l)ase. 
At the top of the base is a small cylindrical compart- 
ment containing a drying agent, silica gel, to prevent 
decomposition of the phosphor by moistui'e. 

Except for a small breather hole to the outside air 
through the drying chamber, the entire instrument is 
sealed. It can be used under severe conditions of rain 
and sea spray, and at the same time can be carried to 
high altitudes without danger of rupture of the cor- 
rector iDlate. Moisture will not even deposit inside a 
cold instrument upon sudden exposure to warm humid 
air. This last advantage is particularly important 
when the metaseopc is used in aircraft. 

A sample instrument weighing 1.8 pounds, includ- 
ing batteries, was submitted to BuShips. As a result 
of tests made by the Navy in September 11)4*^. a pro- 
curement order for 10,000 was placed with the 
Samson United Corporation of TIoehester, New York. 

Pkoductiox-Model Problems 

Although the Type A metascope was compact and 
rugged, it rvas not designed primarily for production. 
Modification of the design to permit large-scale pro- 
duction was undertaken by Eastman Kodak Company 
under Contract OEMsr-1100, and production draw- 
ings were furnished to Samson United Coi’poration. 

The Navy has specified that a red filter be built in 
over the corrector plate, to reduce interference with 
the observe! ’s vision by moonlight or a bright night 
sky. Tliis also eliminates autocollimator action which 
returns visible light in the direction of any infrared 
or visible source viewed with the metascope. A red, 
not an infiared, filter is used to allow final adjust- 
ments inside the metascope to be made easily, looking 
through the filter with visible light. This filter cuts 
the incoming infrared to approximately 80 per cent 
of its true value, but to the dark-adapted eye its vis- 
ible transmission is quite low. 

Most of the production Type A instruments were 
supplied with phosphor powder from the Brooklyn 


Polytechnic Institute, which, under Contract OEMsr- 
982, has improved Standard VI and has also made 
several improvements in the method of production. 
Sensitivities of Standard VI samples increased dur- 
ing the production of Type A, as did the method of 
forming the “'button,’^ the focal surface coated with 
phosphor. Therefore, the average sensitivity of the 
instruments increased. The first instruments had 
threshold sensitivities of 100 nautical-niile-candles 
and the final ones averaged 20 to 25 nautical-mile- 
candles. 

This order of sensitivity rating was set up by Bu- 
Ships as a practical standard of performance. An in- 
strument of 1 nautical-mile-candle sensitivity is one 
with which a fully dark-adapted observer can just 
detect one nautical mile away a specially filtered 
source of 2800 K color temperature (tungsten lamp) 
which has an intensity qf 1 candlepower before filter- 



PiGURE 5. Tyi)e B iiietascoi^e, front view. 

ing. The filter used is an XRX7 type made by the 
Polaroid Corporation, specially selected by BuShips 
foi’ such test work. 

Pcsolving-power tests sliow that the Type A meta- 
scope can resolve one part in 150, or about 7 mils. 

32 2 Type B 

A 414-incb apeitnre ultra ^■iolet telesco})e of approxi- 
mately thi‘ee-power was developed at the same time 
as the 2-inch instrument. Comparison of the sensitivi- 
ties and resolving powers of these two indicated that 
there would be a use for a larger instrument than 
Type A. Accordingly, a metascope was designed in 


DEVELOPMENT OF INSTRUMENTS 


39 


November 1942 with approximately twice the linear 
aperture and twice the weight of the A unit, with 
two-power magnification. This Type B is shown in 
Figure 5, with a scale drawing of the optics in Fig- 
ure 6. It has a clear aperture of 4%, inches, as com- 



pared to 2^/4 inches in Type A, a magnifying power 
of 2.12 and 24-degree field. Like Type A, it operates 
at a speed of //0.55 and gives an erect image. It 
weighs approximately 4.6 pounds. 

As in Type A, two focal surfaces are mounted on a 
swinging fork, so that one can be excited in a side 
pocket while the other is used for viewing. The side 
pockets are each provided with batteries and a filtered 
light source. Standard VI phosphor is regularly used 
in Type B, but another phosphor, B-1, developed at 
Brooklyn Polytechnic Institute, has also been tested 
in this instrument. As seen in Figure 5, two silica 
gel chambers, one with a breather hole, are attached 
diagonally across the exciting pockets. The weight of 
the Type B metascope is so distributed that it may be 
held in one or both hands, using the lower side grips 
over the battery cases shown in the figure. As with 
Type A, unobstructed vision is provided for the un- 
aided eye. 

In the Type B instrument, however, the viewing 
system is quite different from that used in A, since 
that kind of viewing system would be too cumbersome 


for this larger size. Visible light emitted by the phos- 
phor is reflected by the spherical mirror to a diagonal 
mirror, thence through a section of a corrector plate, 
which in this case should be calculated for visible 
light, and finally through the viewing telescope. Erec- 
tion of the image is obtained by a second diagonal 
reflection through a pentaprism in the viewer. This 
gives an extremely compact instrument for one of 
such a large aperture. 

The magnification in Type B should give an in- 
crease in the apparent intensity of a point source of 
4y2 times that of Type A. However, five metallic re- 
flections take place in the B and part of the aperture 
is lost because of the diagonal mirror; the total theo- 
retical gain is thus about 3^2- The magnification will 
produce no change in the apparent brightness of an 
extended source and, due to the many reflections, it 
will actually appear less bright. 

Tests of the range of the Type B were made at the 
contractors laboratory. When a tungsten source of 
400,000 beam candlepower was filtered through 6 
millimeters of Corning No. 2540 glass, a well-dark- 
adapted observer could pick it up with the metascope 
10 miles away on a dark night and 5 miles away in the 
bright moonlight. If a deep red or thin infrared Alter 
were used to cut out the visible light, a well-shielded 
observer could find the lamp at 10 miles in bright 
moonlight. 

An average sensitivity of 7 to 10 nautical-mile- 
candles was obtained in the Type B with Standard 
VI phosphor, approximately three times that of the A. 
It has a resolving power of about 1 in 350, or 3 mils. 

BuShips ordered 5,000 Type B metascopes from 
Eastman Kodak Company, which altered the design 
for production under Contract OEMsr-1100. A red 
filter with a bayonet attachment to fit over the cor- 
rector plate is supplied with the instrument. Produc- 
tion started in 1943 and was completed early in 1944. 

Types O and M 

Two miniature metascopes. Types 0 and M, were 
developed next, using a K-S system of 1%6-ii^ch en- 
trance aperture. The 0 instrument was designed at 
the request of the Office of Strategic Services; and 
the M, which differs from the 0 only in that it con- 
tains batteries and a self-exciting system, was pro- 
duced for BuShips. These were meant to be used as 
pocket instruments in cases where extreme range was 
not desired but lightweight and small size were 
important. 




40 


METASCOPES 


A single, rigidly mounted surface with Standard 
VI phosphor is used in both types. Both instruments 
use a viewing system similar to Type A but with no 
erecting lenses. The phosphor Avas observed with a 
tAVo-elenient eyepiece through a hole in the spherical 
mirror. Type 0 is excited by either daylight or an 
incandescent lamp through the eyepiece, which is 
provided Avith a removable filter of Corning glasses 
No. 428 and medium aklo, for this purpose. Type M 
has a built-in exciting system, using a fixed light 
source placed betAveen the K-S corrector plate and 
mirror, as described for the ultraviolet prototype. For 
emergency use, daylight excitation as in Type 0 is 
also provided. With both 0 and M metascopes, there- 
fore, observation must be interrupted for excitation. 
It Avas expected, hoAvever, that the excitation could 
be done shortly before use, and that the period of 
observation AA^ould be short enough not to require 
recharging. 

In both types, provision is made for an infrared fil- 
ter to be screAved over the corrector plate, and such 
filters are supplied Avith the instruments. Silica gel 
is not used in these metascopes, for they can be pres- 
sure-sealed because of the al)sence of moving parts. 
This also makes the small metascopes more rugged 
than the Types A and B, and less likely to come out 
of adjustment. 

The sensitivity of these small metascopes is approxi- 
mately half that of Type A, giving a threshold value 
of about 40 nautical-mile-candles. Both instruments 
have the serious disadvantage of an iiiA'erted image. 
Types 0 and M Aveigh 4 and 6 ounces, respectively. 

These small instruments, especially Type M, 
aroused considerable interest, ])ut it was felt that a 
better metascope should be made Avhich would give an 
erect image of good quality and have greater sen- 
sitivity. 

32^ Type A-M 

A metascope was designed in the early summer of 
1943 to incorporate the advantages of the miniature 
models Avith those of the larger types; it was desig- 
nated Type A-M (A modified). It employs a single, 
rigidly mounted surface Avith Standard VI phosphor, 
Avith a built-in exciting source. The first A-M used 
the same size K-S optical system as the Type A, giring 
similar sensitivity and resolving power. 

The method of viewing the phosphor results in a 
better image than in any of the previous types. Light 
emitted by it is reflected by the large spherical mirror 
through a small high-index roof pi’ism and a small 


element of a K-S plate in the side of the instrument 
case. There is no eye lens, since the bundles of ra 3 '’s 
are collimated by the spherical mirror and the visible 
image appears at infinity. 

If the Schmidt correcting segment were placed at 
the same optical distance from the mirror as is the 



Figure 7. Type F metascope, front view and rear view. 


large corrector plate (alloAving for the change in 
optical path introduced by the roof prism), the Avhole 
optical system Avould be completely symmetrical, and 
many aberrations Avould be eliminated. HoAveA'er, 
under these conditions the entire available field, as 





DEVELOPMENT OF INSTRUMENTS 


41 


limited by the aperture aud size of the focal surface, 
cannot be seen by the eye, since there is not room 
for a prism large enough to take advantage of the 
whole field. Therefore, to keep the same field in the 
A-M as in the A, the eyepoint is moved closer in by 
making the prism of dense glass and placing the seg- 
ment closer to the mirror. This makes aberrations in 
the image, for the segment is no longer in the proper 
position and off-axis rays traverse the wrong parts 
of the segment correcting curve. Nevertheless, the 
image thus formed is very satisfactory and far better 
than the image in any previous metascopes. Erection 
of the image in one plane is taken care of by the roof 
prism and in the other by an external diagonal mirror, 
which folds to form the cover of the instrument. The 
roof prism need not be made with high optical pre- 
cision as there is no magnification of the image. 

Type A-M weighs about half as much as Type A, 
and takes up only half the space. It, too, is excited 
by internal batteries, but like the M has only one phos- 
phor surface, or button. A metal shutter is provided 
over the eyepiece so that the instrument may be com- 
pletely closed and carried without a case, if necessary. 
Since it has no internal moving parts, the A-M can be 
pressure-sealed, though a compartment for silica gel 
is also provided. The instrument is quite rugged. Ex- 
cept for damage to the shutter or to the arm or hinge 
supporting the diagonal mirror, or actual breakage 
of the corrector plate, little harm can be done to the 
A-M. 

This metascope was seriously considered for i^ro- 
duction. In the meantime, however, the development 
of a new phosphor. Standard VII (see Chapter 4), 
and a new means of exciting it rendered the instru- 
ment, although not the essential design, obsolete. 

32.5 Type A-I 

A new phosphor material, B-1, which is more sen- 
sitive than Standard VI, was developed (see Chapter 
4). In order to take advantage of this, a new pressure- 
sealed metascope. Type A-1, was designed and pro- 
duced by Samson United Corporation. This uses the 
optical parts of Type A, but copies the original pro- 
totype using a single button and a fixed source. The 
sensitivity of this instrument is about 4 to 8 nautical- 
mile-candles. The Navy ordered 5,000 of these instru- 
ments, and delivery has been completed. 

3.2.6 Typg Y 

In May 1943, a very different phosphor was made 
available, which proved to be three times as sensitive 


as Standard VI when excited with ultraviolet light. 
Later experiments showed that this phosphor, which 
gives a visible green response on exposure to infrared 
radiation, can also be excited by alpha particles and 



is then six or more times as sensitive as Standard VI. 
This phosphor has been called Standard VII (see 
Chapter 4). 

It is interesting to note that provision for this type 
of excitation had been made in the original prototype 
telescope, and early experiments had been made with 
a radium source fused into porcelain. 

Because of this new development, another meta- 
scope, Type F, was designed to be used with Standard 
VII. It is %o linear dimensions of the Type A-M 
and uses the radium method of excitation. One of the 
production instruments is shown in Figure 7. Figure 
8 is a scale drawing. Type F was originally provided 
with a removable infrared filter, which was screwed 
into the back of the instrument when not in use. This 
was intended for use on bright nights to exclude vis- 
ible light and enable easier detection of the infrared. 
A later model uses a red filter cemented to the main 





42 


METASCOPES 


corrector plate. The production instrument weighs 
0.7 pound, with outside dimensions approximately 
2 V 4 X 2 V 2 X 3 inches. The optical arrangement is ex- 
actly the same as for Type A-M. 

Excitation Arrangement. Alpha-particle excitation 
is supplied in Type F by mounting a small disk of 
radioactive gold foil, which is usually referred to as 
blitz, upon a lightweight swinging arm within the 
instrument. The radium is contained within, rather 
than on, the surface of the gold, and special precau- 
tions have been taken to minimize the escape of radon, 
which produces objectionable scintillations on the sur- 
face of the phosphor. For excitation, the arm can be 
swung so as to place the blitz about 1 millimeter in 
front of the phosphor surface, or it may be swung 
out of the way behind a barrier in the instrument case 
when the metascope is in use. Even with the blitz in 
the latter position, beta rays will continually strike 
the phosphor, acting as a tricMe charge to keep it ex- 
cited. Once excited. Standard YII will hold its ex- 
citation for many days even in the absence of the 
trickle charge, provided, of course, that the instrument 
is kept closed. Since untiltered daylight quickly ex- 
hausts the excitation of Standard VII, it is very im- 
portant to keep the instrument closed. 

The phosphor may be used immediately after swing- 
ing the exciting arm away, although it continues to 
glow spontaneously for some time. Maximum thresh- 
old sensitivity is obtained after this afterglow has been 
allowed to die dovni. One hour after excitation, or any 
longer period up to at least 2 days, gives the best re- 
sults. For example, if the average Type F is fully 
excited, the threshold sensitivity is 40 nautical-mile- 
candles 5 minutes after excitation, and 6 nautical-mile- 
candles or better 1 hour later. The best Type F shows 
a sensitivity of 2 nautical-mile-candles after 1 hour. 

After a phosphor has been exposed to a strong in- 
frared source for a long time, it becomes exhausted 
and loses sensitivity. For instance, if a surface of 
Standard VII has been exhausted to half sensitivity 
by continuous use during night operations, 1 hour of 
recharging is necessary to bring it up to full sensitivity 
again. However, 10 minutes of recharging will bring 
it back to 80 per cent maximum sensitivity. In both 
cases, maximum sensitivity is not obtained until an 
hour after the excitation is removed. 

Length of Use Before Recharging. It is important 
to determine how long the instrument may be used 
under average operating conditions without interrupt- 
ing its use for a recharging period. The bulk of opera- 
tional use is expected to be conducted at o to 10 


times threshold. However, as a precautionary measure, 
a test was set up with a Type F metascope and an in- 
frared source 100 times the instrument threshold. 
The instrument was clamped in position so that the 
image of the distant source remained fixed on the 
phosphor surface and thus exhausted a very small 
area of about 1/200 of a square millimeter. It was 
found that under this condition, 70 minutes of expo- 
sure were necessary to exhaust this area to one-half 
sensitivity. Under any operating conditions the in- 
strument is, of course, moved, and while most of the 
use is confined to the central areas of the phosphor 
surface, this still involves working areas of as great as 
50 or more square millimeters. Thus, it is probable 
that under any ordinary operating use the phosphor 
cannot be seriously exhausted even by a full night 
of continuous service. Practical experience in the field 
and under combat conditions has borne this out. 

Pressure- Sealing Method. The Type A and B meta- 
scopes were not sealed, but supplied with breather 
holes as already mentioned, because of the fear of rup- 
turing the corrector plate by atmospheric pressure 
changes and also because of sealing difficulties con- 
nected with moving the fork holding the focal sur- 
faces. Subsequent tests showed that the corrector 
plates could withstand pressures up to 25 pounds per 
square inch, so that with the F instrument the only 
thing that prevented pressure-sealing was the move- 
ment of the blitz arm by an outside lever. This was 
overcome by using a sealed push button to release a 
catch holding the blitz arm. When the instrument is 
tilted and the button pressed, the arm falls into posi- 
tion by gravity and is pinned in this position when 
the button is released. All production instruments are 
thus pressure-sealed. Silica gel is inserted in the 
doughnut-shaped cavity behind the spherical mirror. 

All the advantages of Type A-M, good image qual- 
ity, compactness, and lightweight, are included in 
Type F. Two arms are used to support the diagonal 
mirror, making the instrument more rugged than the 
Type A-M, which has only one, but still subject to 
damage while being opened or when left in the open 
position. In view of the fact that the front diagonal 
mirror, which forms the hinged cover of the meta- 
scope, might be injured in operation under conditions 
when the need for the instrument would be critical, 
the design provides a deliberate weak point in the arms 
which normally serve as stops and braces for the mir- 
ror in the operating position. Thus, if the mirror has 
been bi’oken or injured in any way, it is only necessary 
to rip the supporting arms from their stops without 


jnuuiaii-l'J'lTr* 


DEVELOPMENT OF INSTRUMENTS 


43 



doing other dainage. This folds the mirror entirely 
out of the way so that the metascope can be used in 
the fashion of a reflex camera, that is, by looking down 
into the eyepiece while the corrector-plate portion of 
the instrument points forward. The sensitivity is not 
impaired (although the convenience of an erect image 
straight ahead is lost), and the instrument may thus 
be kept in operation under very severe conditions of 
combat service. 

The Type F metascope, with its high sensitivity and 
good image quality, aroused considerable interest. The 
Army Engineer Board placed a total order for 55,000 
instruments with Samson United Corporation and 
with Electronics Laboratories of Indianapolis. Each 
company has designed its own production model ; these 
differ slightly from each other and from the original 
model, but both production instruments are satisfactory. 

3.2.7 Type J 


The Type F metascope is difficult to open with one 
hand and thus cannot easily be used under some cir- 
cumstances, as, for example, by the pilot of a single- 
seater fighter plane. For this reason, the Type J was 
developed at the request of the Navy Bureau of 
Aeronautics, differing from the F only in having a 
stationary diagonal mirror, with a filter and a cover 
that can be flipped down with one finger. It was sub- 
mitted to the Navy but has never been put in produc- 
tion. 


Type H 


Although the F metascope is very compact and so 
can be used in cases where there is no room for a 
straight-through viewing system such as the Type A, 
and although it gives excellent image quality, it is 
somewhat difficult for inexperienced observers to use 
because of the offset optical axis. For this reason, 
another metascope was designed for Bu Ships at the 
same time as the F, called Type H (Figure 9). How- 
ever, it is known by the Navy as Type A-M because 
the designation of the earlier Type A-M had already 
become fixed in certain authorizations. It is also some- 
times referred to as the Type II-AM. 

Type H uses the straight-through optical system 
originally used in Type A, but with linear dimensions 
of the primary K-S system %o those of the A, as 
in the F instruments. It is excited by alpha particles 
in the same manner as the F and has the same aver- 
age sensitivity, but not quite the off-axis image quality, 
since it uses a lens-erecting and viewing system. Type 


Figure 9 . Type H metascope, front view and side view. 


II is pressure-sealed, Avith 2 silica gel chambers on the 
side of the case. The cover can be flipped open, and it 
folds under the instrument Avith a retaining catch. A 
red filter is cemented over the corrector plate. 

Smaller than the A instrument and Avith about half 
its 2-pound Aveight, Type H has four times the sensi- 
tivity, and thus can be used to advantage in any appli- 
cation Avhere the A instrument has been successful. 

Because of the unit magnification, the position of 


44 


METASCOPES 


the image is independent of the motion of the tele- 
scope, and thus two systems can be linked together 
to form a binocular (Figure 10), with no critical 
alignment necessary. This necessitates good adjust- 
ment of the magnification, however, or one image 
might be larger than the other and the two would then 



Figure 10. Type H metascopes used as binoculars. 


be difficult to fuse. A binocular attachment is provided 
with the instruments, so that they can be put together 
in the field in a few minutes. 

BuShips has received 21,000 Type H metascopes, 
and has made an additional postwar request for more. 
The production model in this case is identical with the 
contractor’s original design. 

3 2 9 Type K 

On request from OSRD observers in the European 
area, a small, compact metascope was developed which 
could be attached to the back of a G. I. fiashlight for 
use with airborne troops. The first of these. Type K, 
was taken to the European theater in August 1944. 

With the Type K and its successor. Type L, a new 
principle as applied in metascopes is used, that of a 
solid dielectric optical system. The maximum speed 
that can be obtained in air with a K-S system is 
f/0.50. If a medium of high refractive index n is used, 
the speed is theoretically increased by a factor of n, 
but if the magnification is kept at unity, the sensitiv- 
ity of the instrument remains the same. Increasing the 
speed of the system means that the slope of the K-S 
corrector plate must be steeper by the same factor, 
and this is difficult to accomplish. Therefore, to cut 
off the steepest portions of the curve, the aperture 
diameter of this solid system is made slightly smaller 
than that of an air system. Type K has a speed of 
//0.36 when glass of index 1.532 is used. 


The great advantages of a solid system are that it 
can be treated very roughly, since the assembly is 
extremely rugged, that smaller sized units can be made 
than are feasible with an air system, and that perfect 
optical symmetry can be obtained, resulting in an 
image with no aberrations for axial rays. 

Solid K-S systems had been used with ultraviolet 
phosphors in 1941, but not applied to the infrared 
until the need arose. The optical parts of the first such 
metascope. Type K, consist of three cemented glass 
elements and an eyepiece. The back element has two 
spherical curves, one of which is polished and alumi- 
nized to serve as the spherical mirror. The other has 
a ground surface in the focal position. To this is 
cemented a thin separator plate with a hole in the 
center, of the same diameter as the focal surface, to 
facilitate coating this with phosphor material. After 
the phosphor has been applied, the front element, 
which has the aspheric correction curve on the outside, 
is cemented on. The back surface is aluminized except 
for a small hole in the center, through which the phos- 
phor is viewed by means of a three-element eyepiece. 

This metascope gives an inverted image of rather 
good quality over most of the field; however, it has 
all the distortion of the Type 0 and M units, and the 
eyepiece aberrations and backward curving field make 
the image very bad at the extreme edge. When mounted 
in an aluminum case, it is about inches in depth 
and IV 2 inches in diameter. It has never been con- 
sidered for production because of the far superior 
quality of Type L. 

3.2.10 Type L 

A solid form of metascope giving an erect, nondis- 
torted image was achieved in the spring of 1945 in 
the Type L unit, developed to meet the specific re- 
quirements of extreme compactness with erect image 
prescribed by the Army Engineer Board, Fort Belvoir, 
Virginia. Photographs of this instrument are shown 
in Figures 11 A and IIB, and a scale drawing in 
Figure 12. 

In the Type L metascope, the front element is di- 
vided into two parts, as shown in the drawing. When 
cemented together, parts B and E act as in the Type 
K, correcting for spherical aberration in the infrared 
image cast on the phosphor. The back element D and 
the separator plate C have the same function as before. 
The method of viewing the phosphor, however, uses 
somewhat the same principle as does Type F. A 45- 
degree wedge is cut out of the front element and in 


DEVELOPMENT OF INSTRUMENTS 


45 


its place a viewing segment E is cemented. This seg- 
ment is a portion of another front element, and has 
an aluminized ellipse on its hypotenuse face. This 
acts as a diagonal mirror to reflect the visible light 
coming from the spherical mirror to the eye ; the exit 



Figure 11 A. Type L metascope with carrying case. 



Figure IIB. Close up of Type L, showing roof prism. 

face of the segment corrects for spherical aberration. 

Complete erection of the image is obtained by means 
of a roof prism, A, mounted separately from the rest 
of the system to receive the incident light. A red 
filter F is placed between the roof prism and the K-S 
system. 

Type L has a threshold sensitivity of 5 to 6 nau- 
tical-mile-candles, comparalile to the 5 nautical-mile- 
candles for the average F and H. It has no axial 
aberrations in its image ; the correcting surface of the 
eyepiece segment is in its proper position, and the sys- 
tem is completely symmetrical. Due to refraction in 
the glass, an extremely wide field of 45 degrees is 
obtained. It has an aperture of 23 millimeters and a 


weight of 3 ounces, compared to 12 ounces in the size 
F. The carrying case is designed to screw directly 
onto a G. I. flashlight. Both the housing and carrying 
case are made of Lucite. 

Standard Vll-b phosphor is used in the Type L, and 
was developed expressly for this service. Yll-b is very 
similar to VII in most respects, but differs in permit- 
ting regeneration in powder form without loss of 
sensitivity. It also permits a sensitivity on ultraviolet 
excitation which is substantially that obtainable by 
Standard VII on radium excitation. This makes pos- 
sible the excitation of the instrument by daylight, 
and a suitable ultraviolet transmitting filter is 
mounted in the carrying case of the instrument to 
accomplish this. By careful attention to the character 
of the filter, which is so positioned in the case that it 
always registers with the eyepiece of the instrument, 
it has been found possible to get full excitation of the 
instrument in about 5 minutes^ exposure to average 
blue sky light, while 20 to 30 minutes’ exposure is re- 
quired with a very dark overcast sky. Prolonged ex- 
citation has no known deleterious effect, so that the 
instrument can be left exposed all day if so desired. 



Figure 12. Assembly of Type L; A, roof prism; B, 
front element; C, separator plate; D, back element; 
E, eyepiece segment; F, red filter. 


Standard Vll-b phosphor has the same fortunate 
properties of Standard VII in retaining its excita- 
tion for many days and in storing a sufficient amount 
of energy to permit all-night use without recharging. 

Although the prototypes of the Type L instrument 
have been made in glass, the Polaroid Corporation 
undertook to investigate the possibilities of plastic 
construction. Using a drop-molding process similar 
to that developed for the Kellner-Schmidt glass cor- 
rector plates (Section 3.4.1), the University of Roch- 





46 


METASCOPES 


oster prodnc'ed glass molds to permit polymerization 
casting of the aspheric front element for the Type L, 
and these molds have been furnished to the Polaroid 
Corporation. The latter has applied this method very 
successfully, and the Army Engineer Board placed an 
initial contract for manufacture of a few instruments 
by that company. The Engineer Board has also placed 
a parallel contract for manufacture of the glass ver- 
sion by the Eastman Kodak Company. 

Stadiameter Attachments 

A stadiametric device for the Type A metascope 
has been designed to enable an observer to determine 
the range as well as the direction of an infrared 
source. The stadiameter is shown schematically in 
Figure 13 and photographs of the assembled instru- 
ment in Figure 14. 

Two mirrors, A and B, are mounted over the meta- 
scope aperture with mirror B obscuring half of the 
aperture. If A and B are parallel, the light received 
by the metascope through the mirror system will be 
parallel to that received through the unobstructed 
aperture, and a single image will result. If, however, 
A is slightly tilted with respect to B, a double image 
will appear. Conversely, by tilting A, two separated 
images can be brought into coincidence, and, if the 
horizontal separation of these sources is known, their 
distance can be determined by the angle through 
which the mirror must be moved. A calibrated screw, 
provided with a small, dark-red light source and bat- 
teries to enable reading at night, gives the angular 
displacement of mirror A. 

xAlthough the resolving power of the Type A meta- 
scope is only 5 mils, the stadia setting can be made to 
an accuracy of approximately 1 mil. This is because 
one mirror can be slightly tipped by means of a cam 



I 

E) !» 

Figure 13. Stadiameter for Type A, schematic drawing. 

so that instead of the images being brought into co- 
incidence, they are lined up one above the other and 
their center lines matched. 

Since the stadiameter is attached on the outside of 
the metascope, it is unaffected by any distortion due to 
the metascope itself, although the instrument should. 


of course, be kept in focus. When set at zero, the stadia 
attachment is neutralized, giving 95 per cent of full 
image brightness. The stadiameter weighs 0.8 pound, 
including the battery. It can be attached or removed 
in the field in less than 5 seconds by means of a bay- 
onet clamp. 

A similar attachment on the Type B metascope 
would be far too bulky. However, since the Type B 

' ' "'“1 



1 3 4 5 6 


Figure 14. Stadiameter for Type A, front view and 
rear view. 

has a completely symmetrical optical system, up to 
the two-power telescope, the stadiameter can be in- 
corporated within the instrument without fear of 
sacrificing accuracy. By splitting the cemented doublet 
of the telescope into two halves and moving these par- 
allel to their cut surface, one image may be split into 
two parts, or two images brought into coincidence. 
As in the Type A stadiameter, allowance is made for 
displacing the images vertically to allow greater ac- 
curacy of setting. The Type B metascope, complete 
with stadiameter, is shown in Figures 15A and 15B. 
The scale is made of Lucite and lighted by pressing a 
button on the side. Type A stadia attachments have 
been put into production, but not the attachment for 
the Type B. 





APPLICATION OF PHOSPHORS TO METASCOPES 


47 



Figure 15A. Stadiameter for Type B, attached to 
metascope. 



Figure 15B. Stadiameter for Type B, disassembled 
parts. 


33 APPLICATION OF PHOSPHORS 
TO METASCOPES 

Infrared-sensitive phosphors are thoroughly de- 
scribed in Chapter 4. A brief discussion of the prop- 
erties as applied to use in metascopes will be given 
here. At present it is by no means possible to predict 
and control all the desired phosphor properties, A 
wide variety of emission spectra has been made avail- 
al)le, because of the preparation and study of many 
thousands of phosphors. Stimulation spectra have been 
obtained with maxima out to 1.3 microns, and it is 
desirable to push the stimulation to still longer wave- 
lengths, or greater intensities at the 1.3 micron limit. 
The excitation spectra, afterglow, spontaneous decay, 
and quantum efficieficy are controllable only in part; 
resolving poiver can be partially controlled by the 
grain size and thickness of the phosphor layer. 


^ Optical Properties of Standards 

Starting in July 1941, research was undertaken at 
the Institute of Optics in the development and pro- 
duction of high-sensitivity infrared phosphors. Orig- 
inally, the purpose was to reproduce two types of 
phosphors that had been made by Urbach and Kunz 
in Vienna in the 1930’s. This was virtually accom- 
plished by the beginning of 1942, and since then new 
phosphors have been made which are either more sen- 
sitive or more adaptable for use. 

All these phosphors are composed of a basic mate- 
rial, a flux and one or more activators. After prepa- 
ration, a phosphor may be regarded as containing a 
matrix of base and flux in which is embedded a very 
small amount, about one part in ten thousand, of 
activating impurities. The energy levels of the matrix 
and activators are mutually perturbed, and quasi- 
stable states are produced which provide traps for 
excited electrons. Upon infrared stimulation, the elec- 
tron receives enough energy to release it from the trap 
and falls back to the ground state with the emission of 
visible light. 

Standard VI, the phosphor used in Types A and B 
metascopes, uses a strontium sulfide base and equal 
amounts of samarium and europium activators. Stand- 
ard VII has the same base, but cerium and samarium 
activators. The B-1 phosphor, developed at Brooklyn 
Polytechnic Institute, uses samarium and europium 
activators, as in Standard VI, but a strontium sel- 
enide-strontium sulfide base. 

A careful study of two-activator phosphors has been 
made, and as early as May 1942 several facts became 
evident. Of the two activators, one, the dominant, 
determines the color of emission while the other, 
the auxiliary, determines the stimulation spectrum. 
Changing the basic material, although in most cases 
secondary to changing the activators, will also cause 
a shift in both emission and stimulation spectra. Al- 
though the excitation spectrum is primarily deter- 
mined by the dominant activator, it will also change 
with the basic material and possibly with the flux 
used. 

The threshold sensitivity of a phosphor at a given 
time will depend not only on the color of emission 
but also on the brightness of the afterglow and the 
stimulability at that time. Stimulability is a measure 
of the efficiency of conversion of infrared radiation 
into visible light. Since these two factors change with 
time, good sensitivity will occur when the background 
is sufficiently low and the stimulability still sufficiently 




48 


METASCOPES 


great. Standard VI decays to practically no back- 
ground in 10 minutes after excitation, while Standard 
VII shows an observable background after 18 hours. 
However, Standard VI loses much of its stimulability 
in a few minutes, and Standard VII shows little loss 
of sensitivity a week after excitation. In spite of its 
background. Standard VII is far preferable to Stand- 
ard \1 after a period of 2 minutes or more. While the 
B-1 is much better than VI, it is considerably less sen- 
sitive than radium-excited Standard VII. Neither B-1 
nor Standard VI shows good sensitivity when radium- 
excited. 

The dominant factor determining the threshold 
sensitivity is the quantum efficiency of a phosphor. 
This depends upon the amount of light the phosphor 
absorbs and the ability of the phosphor to turn in- 
frared energy into emitted visible light. It is quite 
possible for one phosphor to absorb less infrared en- 
ergy than another, but to make more efficient use of it. 
For example, although Standard VII absorbs only 
4 to 6 per cent of incident infrared when excited, 
it is much better than Standard VI, which absorbs 
50 per cent. 

3.3.2 Physical Properties of Standards 

It is a common property of all phosphors that they 
lose sensitivity when subjected to grinding. Thus, some 
form of regeneration must be undertaken. The first 
method was successive pulverization and reheating to 
just below the sintering temperature. It was later 
found that a single heating at a lower temperature 
for a longer time will produce the same results. 
The addition of magnesium oxide to the phosphor 
before baking, suggested by the New Jersey Zinc Com- 
pany, results in a much softer cake, which can be 
crumbled into a powder with less destruction of 
luminosity. If extremely fine powders are desired, 
the magnesium oxide combination can be ground, then 
regenerated, and the final powder sprayed or painted 
on the desired surface. 

Even after obtaining a fine powder, difficulties were 
sometimes still encountered in trying to apply the 
powder to a focal surface. Spraying the regenerated 
phosphor on focal surfaces resulted in irregular spots 
and thicknesses at first, although the powder obtained 
by the magnesium oxide method can be sprayed. An 
attempt to mold buttons and machine them to the 
proper thickness was successful in the case of Standard 
VI, while Standard VII is sprayed on a ground back 
and regenerated in place. 


Carbon has been chosen as the backing substance 
for buttons, because it combines many essential qual- 
ities. It does not warp or react with the phosphor even 
at high temperatures; it is easily machined and it 
will absorb, not refiect, any light that reaches it, thus 
increasing the resolving power. 

The phosphors as now formed on buttons are not 
injured by extreme changes in temperature, and they 
withstand any shock that the metascopes themselves 
will stand. However, they must be protected from 
water vapor, which causes marked deterioration of the 
sulfide by hydrolysis. For this reason silica gel is 
supplied in all instruments except the solid systems, 
and in every case possible the instruments are pres- 
sure-sealed. 

^ Methods of Excitation 

Excitation filters should be chosen with great care 
to exclude any radiation that may stimulate or quench 
the phosphor. Standard VII was first found to be 
excitable, by the ultraviolet, with such an exhaustion 
band occurring in the region of maximum excitation. 
This causes the phosphor to be partially exhausted 



Figure 16. A. Sectional view of mold and blank be 
fore heating. B. Appearance after temperature cycle. 
C. Finished plate edged to diameter, lower surface 
ground and polished flat. 


rjidun i jiii HJLtr 


PRODUCTION OF ASPHERIC CORRECTOR PLATES 


49 


during excitation, resulting in values of stiniiilability 
less than saturation. Alpha-particle excitation, on the 
other hand, does not exhaust the phosphor nearly as 
much, for reasons still not understood. 

The scintillations produced by alpha particles from 
radon and its decay products, however, are a problem 
when blitz excitation is used. Such scintillations are 
confusing to an obsei ver because they are hard to dis- 
tinguish from small distant light sources. The prob- 
lem in this case was solved by keeping the radon with- 
in the foil by a coating of fused silver chloride. 

Exposure of the phosphor to too long and too in- 
tense a source of radium causes a marked deteriora- 
tion of the phosphor. For this reason the radium con- 
tent of the blitz foil must be carefully regulated. 
Suitable protection must be given by the housing to 
shield the person using the metascope from harmful 
radiation. This is easily accomplished in the case of 
alpha and beta particles, and the gamma radiation 
from the small amount of radium present in any one 
instrument is of too low intensity to be a hazard 
under ordinary operational use. 

34 PRODUCTION OF ASPHERIC 
CORRECTOR PLATES 

The aspheric surface requiied on the Kellner- 
Schmidt corrector plate cannot be produced by ordi- 
nary production optical grinding and polishing tech- 
niques, so that in the past the only method of making 
such plates was to grind and polish the surface by 
hand. This is a very laborious and time-consuming 
operation that requires a highly skilled optician and 
renders the method unsuitable for mass production. 
Late in 1941, a study was begun of the possibility of 
producing corrector plates by the process known as 
dropping. Essentially, this method consists of heating 
a plane-parallel glass disk, which is optically polished 
on the upper surface, until it becomes sufficiently plas- 
tic to sag or drop into a mold of predetermined form. 
Parabolic reflectors of searchlight quality have been 
produced in this fashion for many years, but the 
process was not refined to the point where aspheric 
surfaces approaching ophthalmic quality could be 
obtained. 

^ The Dropping Process 

From the outset, the molds have been made from a 
refractory mateiial called greenhlocl' ,miim\iRctmed by 
the American Optical Company. This greenblock ma- 


terial is somewhat abrasive and difficult to machine, 
but has the desirable properties of not warping in the 
required temperature range and of lasting for 100 or 
more firings. 

The aspheric curve on the greenblock was first cut 
on a lathe in a series of steps, from a table of coordi- 
nate data giving the depth, Y, at equally spaced in- 
crements, A.r of radius .t. A.r was chosen to be a mil- 
limeter or less so that successive values of Y did not 
differ by more than 0.1 millimeter in any region of the 
curve. These steps were then smoothed out by hand 
with emery paper until the tool marks almost dis- 
appeaied. Later in tliis development, the molds were 
cut on one of two contouring lathes specifically de- 
signed to reproduce the curve from a precision metal 
template, described in Section 3.4.2. 

Considerable experimentation was required to de- 
termine the proper heating cycle. It was found that 
too high a temperature caused devitrification of the 
polished glass suiface, while too low a temperature 
did not permit the glass to sag completely into the 
mold, even when the maximum temperature phase 
of the heating cycle was greatly prolonged. The dif- 
ference in temperatures producing a good or bad 
suiface amounted to about 30 C. Figure 16 shows the 
steps required to obtain a corrector plate from the orig- 
inal disk of glass. It should be noted that no work 
is done on the polished side after dropping; once a 
plate is dropped, it is only necessary to grind and 
polish the back flat and edge it to the proper diameter. 
All dropping has been done in electric furnaces pro- 
vided with precision temperatuie controls. A new 
glass, developed by the American Optical Company 
and manufactured by the Pittsburg Plate Glass Com- 
pany, called 1045x, was adopted in 1943 and is now 
the standard glass used for this purpose. 

Suction Molding. Gravity dropping did not always 
insure positive enough contact between the plate and 
the mold, and tangential flow of the glass was difficult 
to control. In 1943, it was decided to try a partial 
vacuum between the two. Therefore eight No. 46 holes 
were drilled through the mold around the zone where 
the glass touches last. The mold was then placed on 
an Inconel platform, inside the furnace, which was 
connected to an ordinary domestic vacuum cleaner 
outside. The platform made from this heat and cor- 
rosion-resistant alloy was designed so that vacuum 
connections could be made with several molds simul- 
taneously, for multiple dropping. With this arrange- 
ment, as shown in Figure 17, a pressure differential 
of 48 mm llg was produced between the mold and 




50 


METASCOPES 


upper glass surface; the large eapaeily uf the vaeiiuiu 
cleaner, even when operated below rated speed, is 
adequate to handle the very small amount of leakage. 

Suction molding greatly reduces the total time for 
a heating cycle. Whereas with gravity dropping, the 



Figure 17. Inconel fixture in furnace for multiple 
dropping. 


glass had to he kept at maximum temperature for an 
8-hour period, only 25 minutes at maximum tempera- 
ture are required under vacuum; the first 15 allow 
for the equalization of temperatures throughout the 
furnace, and the vacuum is turned on for the remain- 
ing 10 minutes. Electric time clocks turn the vacuum 
on and off at the proper times. This cycle has been 


1015x glass now used Jciigthens the heating cycle 
about one hour. 

Correctoe-Plate Tesung 

A dropped plate can be tested in one of two ways: 
by a direct measurement of its coordinates, or by an 
examination of its optical performance in the final 
system. Both procedures take about the same amount 
of time, but the second is employed because it is by 
far the more accurate and has the advantage of pro- 
viding information about image quality directly. 

The testing unit shown in Figure 18 consists of a 
spherical mirror and a smooth, diffusing focal surface 
which is viewed by a microscope equipped with a 
micrometer eyepiece. The plate is positioned at the 
mirroPs center of curvature and the entire unit placed 
to receive light from a point source either at infinity 
or a finite distance, depending upon the particular 
application. Before proceeding with the testing, the 
plate is centered with respect to the mirror by moving 
it in its own plane. When this adjustment is correct, 
the out-of-focus image formed on the focal surface is 
a bright concentric circular pattern usually containing 
one or more bright rings caused by zonal errors. Any 
departure from concentricity which cannot be elim- 
inated by recentering indicates that the plate is not 
a true surface of revolution. 

The next step is to cover the entire plate except for 


PARABOLIC 

MIRROR 



CORRECTOR PLATE 
OILED TO PLANE- 
PARALLEL PLATE 



V 




FOCAL 

SURFACE 


SLIDING 

APERTURE 


Y\ KELLNER-SCHMIDT 
\\ SPHERICAL MIRROR 

\\v 



MICROSCOPE WITH 
FILAR MICROMETER 
EYEPIECE 


Figure 18. Kellner-Schmidt testing unit. 


adopted as standard except for variations in the maxi- 
mum temperature to suit the type of glass. Even with 
the same type, however, the maximum temperature 
is selected with a 10-degree C range, depending upon 
the maximum slope and second derivative of the par- 
ticular mold curve. In all cases, the lowest possible 
temperature is used which produces barely visible 
markings of the suction holes on the back of a dropped 
plate. Except for plate diameters above 5 inches, no 
annealing of the glass is performed. Annealing the 


a small aperture which can move in front of the plate 
along a diameter in the manner of a Hartmann test. 
A perfect plate would focus all rays at the same point 
and, consequently, no image displacement would be 
noted as the aperture is moved. Any displacement 
which appears is almost exactly proportional to the 
error in slope at the zone under investigation. These 
displacements are measured at small intervals and 
averaged for corresponding zones equidistant from the 
center. This visual scanning is usually performed with 


PRODUCTION OF ASPHERIC CORRECTOR PLATES 


51 


light in the socliuni I) line region. If the plate is to 
be used in some other spectral region, the data are 
adjusted to that region by adding a displacement cor- 
rection proportional to the slope at each zone to com- 
l^ensate for dispersion. The corrected displacements 
are numerically integrated from the center to edge 
and the error in sag, AF, at any zone, Z, is found by 
multiplying the integral at that zone by a factor which 
depends chiefly upon the focal length of the system 
and the index of the plate, and to a much smaller 
extent upon the slope of the surface. For practical 
purposes it can be considered a constant. 

Mold-Sukface Curves 

No mention has yet been made about the shape of 
the curve on the corrector plate with respect to that 
on the mold. Because of the finite thickness of the 
glass and unequal expansion coefficients of glass and 
greenblock, the two curves are not the same. No fun- 
damental study of the difference in the curves has been 
undertaken; instead, the first approximation in mak- 
ing a corrector plate is to cut the desired curve on the 
greenblock. In the first dropping, the gross differences 
AF between the plate and mold curves appear and 
are determined from an optical scanning test. The 
values of AF are usually a small fraction of the total 
sag F and vary so slowly across the plate that mod- 
ifying the mold curve diiectly by AF results in very 
nearly the same changes in the sag of the plate. After 
five or six trials, the method of successive approxi- 
mations yields plates of a quality approaching the 
reproducibility of the dropping process. 

Extensive experience in developing mold curves 
proves that the zone having the maximum curvature 
is the most difficult to control. In the case of Iv-S 
corrector plates, this region is generally at the edge 
of the clear aperture, which further increases the diffi- 
culty since a definite edge effect is noticed due to the 
boundary. Whenever possible, to counteract the edge 
effect, a glass plate considerably larger in diameter 
than the clear aperture needed is dropped. 

Sizes Made by Droppixg 

K-S corrector plates have been made by dropping 
in a variety of sizes from 0.9 inch to 9.5 inches in 
diameter. Two of the designs are used as projection 
systems of slower speed than the metascopes (Sec- 
tion 7.2.1 in Chapter 7). Angular resolution of the 
//0.55 systems is roughly % as good as with the 
slower types. This limitation is undoubtedly due to 
the fact that the maximum plate slope for the //0.55 


is of the order of 0.17, whereas it is only 0.03 for 
speeds around //O.8. 

The dropping process was also applied to produce 
reverse Schmidt curves out of glass, which in turn 
served as molds for making plastic corrector plates by 
a casting process. In this work, the glass curve was 
developed until it closely approximated the coordinates 
given in the original optical calculations. From that 
point, scanning tests performed upon plastic castings 
were used to make the final corrections to the green- 
block curve. 

^ ^ Cutting Greenblock Curves 

As the extent and importance of the aspheric-plate- 
dropping program developed, it became necessary to 
design and construct contouring machines for cutting 
greenblocks. One of these uses a l-to-.l ratio between 
template and duplicate. It is capable of turning disks 
as large as 13 inches in diameter and of reproducing 
templates with sags as great as 2 inches. Design work 
on this machine began in the last month of 1942, and 
the first trial cuts were made in February 1944. An- 
other machine, with a 5-to-l ratio between template 
and work, was constructed before the 1-to-l, and is 
used for all work less than 3 inches in diameter. 

1-TO-l COXTOURIXG MaCHIXE 

The contouring machines must be able to cut the 
extremely abrasive greenblock material as well as fer- 
rous and nonferrous metals; it is still possible, how- 
ever, to use a stationary tool and not an abrasive 
grinder as would be necessary for glass. In order to 
take full advantage of the highly accurate aspheric 
metal templates available, the system must be one of 
high inherent accuracy and the entire construction 
extremely rigid and precise. In addition, some type 
of template corrector mechanism is desirable to im- 
prove the accuracy of the master template where 
necessary, to eliminate constant but predictable ma- 
chine errors such as wear of the tool, and to allow 
a small amount of curve alteration to be carried out 
during work with experimental templates. 

Figures 19A and 19B show the 1-to-l contouring 
machine as finally constructed. A standard bench- 
lathe is used as a base, supplied with raising blocks 
to allow a diametral capacity of 13^2 inches. The drive 
is equipped with a speed-selector unit to allow a range 
of spindle speeds from 30 to 300 rpm. 

The tool-carriage system consists of a parallelogram 
with two parallel movable arms anchored at one end. 




52 


METASCOPES 


by means of preloaded self-aligning ball bearings, to a 
heavy duty cross slide. At the other end, the arms are 
attached in an identical manner to a traveler head, 
which carries the cutting tool and the template- 



Figure 19A. 1-to-l aspheric contouring machine. 
Side view. 


follower mechanism at opposite extremities. The move- 
ment of the cross slide carries the traveler head across 
the master template, causing the cutting tool to trace 
the template curve across the face of the work mounted 
in the spindle. The cross slide is provided with an 
automatic variable-speed power feed to insure smooth 
surface cuts on any material. 

The geonieti’y of the system and the character of 
the aspheric surface generated require that the tool 
and follower both be sections of circles of identical 
radius. The follower was made in the shape of a hemi- 
sphere of 0.250-inch radius and mounted on a long 
finely threaded screw fitted with a calibrated dial to 
allow accurate feeds to be obtained. Since the tool was 
then required to also have a 0.250-inch radius cutting 
form, it was decided that a circular tool of 0.5000-inch 
diameter, mounted on a vertical axis, would fulfill the 
form requirements and could be rotated to present a 
new cutting surface when necessary. A tool of this 


type, constructed out of tough steel with a Carboloy 
disk brazed on to form the cutting edge, was found 
to be entirely satisfactory. 

In order to provide continuous corrections to the 
master template curve during the generation of a sur- 
face, provision was made in the machine for mounting 
a second metal template next to the axis of the fol- 
lower adjustment screw. As the traveler head moves 
across the work, it traverses the two templates at the 
same time. The accuracy of this secondary template 
need not be very great, as the demagnification of the 
correction applied by it is 200 to 1. The range of sag 
correction supplied by this mechanism is plus or minus 
0.010 in. With corrector templates, the various ap- 
proximations to the proper greenblock curve can be 
made as described above. In case the master template 
is not quite accurately centered, the corrector template 
can be cut to correct this; the corrector also provides 
for gradual wear of the tool during the cut. Both the 
master and corrector templates are cut from hard 
Vs-inch sheet-aluminum alloy and the two types are 
uniform in size and reference surfaces. 

(Ireenblock chucks for this contouring machine 
were constructed in such a manner as to allow the 



Figure 19B. l-to-l aspheric contouring machine. 
Top view. 


blocks to be quickly mounted or dismounted and also 
to permit accurate centering. 

Some 200 greenblocks in sizes up to 12 inches in 
diameter, a great number of metal test blocks, and 
several Lucite corrector plates have been cut on this 
machine and all proved to be of acceptable quality. 
Measurements of all these curves indicated that the 
machine has a systematic erior or departure from the 






PRODUCTION OF ASPHERIC CORRECTOR PLATES 


53 


curve desired of less than 0.001 inch, and that it will 
duplicate curves to better than 0.0001 inch for any size 
of template within the capacity of the machine. 

5-to-I Contouring Machine 

The 5-to-l contouring machine, used for cutting 
all greenblocks less than 3 inches in diameter, uses a 
different system than that of the 1-to-l. A long, 40- 
inch arm is anchored at one end in crossed pivots so 
that it is free to swing in any direction about this pivot 
point, but is prevented from rotating about its own 
axis. At the opposite end of the arm is the template 
follower, with the cutter at an intermediate point. 
The work revolves continually, at speeds up to 3,000 
rpm. As with the l-to-l machine, if a spherical cutter 
is used the follower must also be a segment of a sphere. 


The same tool as used in the 1-to-l machine has also 
proved successful here. 

In operation, the arm is so counterweighted that 
the follower rests lightly but firmly on the template. 
As the arm is swung through a horizontal arc across 
the template, it also swings through a small vertical 
arc as the sag of the template surface increases and 
decreases. The cutting wheel traces out a reduced 
replica of the path of the follower and, as the work 
revolves, cuts a reduced replica of the template sur- 
face on the greenblock. No corrector template is 
necessary. 

By October 1943, the machine was capable of cut- 
ting greenblocks to a total error spread of 0.008 mil- 
limeter or 0.0003 inch. Since then approximately 150 
greenblocks have been cut on the 5-to-l machine. 


Chapter 4 


INFRARED-SENSITIVE PHOSPHORS 


By Franz Urbach and Mary Banning^ 


41 INTRODUCTION 

T he development of infrared-sensitive phosphors 
capable of emitting visible light when exposed to 
infrared radiation was undertaken by a number of 
laboratories under OSRD contract from 1941 through 
August 1945. All the various contractors worked close- 
ly together during this period and were kept well- 
informed of the progress made in each division. Since 
the advances made in one laboratory were often chief- 
ly due to a suggestion coming from another, there will 
be no attempt made in this chapter to separate the 
work of the several laboratories. In July 1941, devel- 
opment started at the University of Rochester under 
Contract OEMsr-81, and in 1943 other groups joined 
the investigation from the Polytechnic Institute of 
Brooklyn (OEMsr-982), the General Electric Com- 
pany (OEMsr-1155), the New Jersey Zinc Company 
(OEMsr-740), and the Radio Corporation of Amer- 
ica (OEMsr-440). 

Early attempts to use phosphors for the detection of 
infrared signals were confined to the use of materials 
with a high phosphorescent afterglow that was extin- 
guished when exposed to infrared radiation. This nec- 
essitated frequent re-excitation or the continual mo- 
tion of the phosphor surface. Until 1934 no phosphor 
showed sufficient sensitivity of emission upon infrared 
stimulation to be used for any practical purpose. The 
first practical infrared-stimulable [IRS] phosphors 
were made by a group of Viennese scientists during 
the 1930^s. They developed two kinds of phosphors, 
one for use at dry-ice temperature with a green emis- 
sion, and one for room temperature use with a red 
emission. These two phosphors were the background 
for the greater part of OSRD work in this field. 

Every IRS phosphor is composed of a basic matenal, 
a flux, and a very small amount of one or more ac- 
tivators. Variations in any one of these or in the 
method of preparation of the phosphor will change the 
characteristics of the resulting material. The leinain- 

‘‘Both of the University of Rochester until December 1945, 
when Dr. Urbach joined the staff of the Eastman Kodak 
Company. 


der of this section is devoted to the definition and dis- 
cussion of the desirable phosphor characteristics. In 
Section 4.2, the general development and properties 
of the most important phosphors are given and the 
relationship of the various components discussed. Sec- 
tion 4.3 deals with the chemical methods of prepara- 
tion and mechanical means of forming smooth, pre- 
cisely shaped surfaces, and Section 4.4 with the meth- 
ods of observing phosphor characteristics. Section 4.5 
is a brief summary of the theory of such IRS phos- 
phors. 

4.1.1 Properties of IRS Phosphors 

In the field of phosphorescence, no strict definitions 
or terminology have ever been agreed upon, so the 
following definitions are inserted to make the suc- 
ceeding discussion clear. 

An infrared-sensitive phosphor, unlike the more 
common ultraviolet type, requires excitation prior to 
use. Exciting radiation, which may be visible, ultra- 
violet, X rays, cathode rays, or alpha particles, lifts 
electrons within the phosphor from some ground state 
to a higher level. Some electrons immediately return to 
the ground state, causing luminescence, while others 
fall into various traps; the phosphor is then said to 
contain excited states. Part of these trapped electrons 
will spontaneously return to the ground state with 
time; this spontaneous emission occurring after ex- 
citation is called the aftergloiv or background. This 
return to the ground state is temperature dependent 
and in some cases can be accelerated by infrared 
radiation, in which case it is called stimulation, 
appearing as a flare-iq) of the emission of the excited 
phosphor. 

The total amount of light energy which an excited 
phosphor is capable of emitting is called its light sum. 
Both the brightness of the emission and the rate of 
decay of a phosphor are determined by the rate of 
release of this stored energy. In many cases some in- 
frared or other radiation diminishes the brightness 
of a phosphor without accelerating the emission, and 
is called quenching. Extinction, the net result of loss 


54 


Ul i MWHWWW feP 


PHOSPHOR DEVELOPMENT 


55 


of brightness by either stimulation or quenching, is 
distinguished from exhaustion, which is due to stimu- 
lation alone. The ratio of stimulated brightness to the 
intensity of the stimulating radiation is called the 
stimulabilUy of the phosphor. A gradual growth of 
stimulated brightness during constant infrared ir- 
radiation is called the inertia of stimulation, while a 
persistence of emission after the end of stimulation is 
called the time lag. 

In order to be suitable for military applications, 
particularly for infrared detection, a phosphor must 
fulfill a complex set of conditions, some essential, some 
desirable, depending on its use. Of primary importance 
is that the phosphor have a high quantum efficiency, 
i.e., that as much as possible of the incident energy is 
turned into visible emitted energy. This requires that 
the absorption of infrared by the phosphor be high 
and at the same time that the most efficient use be 
made of the amount absorbed. Some phosphors may 
absorb a large amount of infrared but make inefficient 
use of it, while others absorb little but use almost all 
of this. 

The background or afterglow at the time of use 
should be low, but for many purposes a faint back- 
ground is desirable. The emission spectrum should 
match the sensitivity curve of the dark-adapted (sco- 
topic) eye as closely as possible, while the stimulating 
infrared spectrum should extend to as long wave- 
lengths as possible to avoid detection by image tubes 
or other devices used by the enemy. Excitation should 
be accomplished by daylight or an easily available and 
portable source permitting the attainment of maxi- 
mum sensitivity within a short time. 

Sensitivity to infrared radiation should persist for 
a long period after excitation, requiring that the spon- 
taneous return of the trapped electrons to the ground 
state be slow. In order that sensitivity be exhausted 
slowly when in use, the useful amount of energy 
stored, the light sum, should be large. Both the inertia 
and the time lag should be small in order to permit 
the detection of fast signals and moving objects. 

Physically, the phosphor should be capable of being 
formed into smooth precise surfaces as for the but- 
tons, described in Chapter 3. Its resolving power 
should be high to enable the detection of closely spaced 
multiple lights or the details in a scene. The surfaces 
should withstand severe mechanical shock, high hu- 
midity, and sudden changes in temperature. Finally, 
the infrared sensitivity of the phosphor should be con- 
stant over all possible temperatures of use. 


4 2 PHOSPHOR DEVELOPMENT 

4.2.1 Early Phosphors, Standards 

I through V 

By the end of 1941, the two Viennese phosphors 
had been reproduced successfully under OSRD con- 
tract. The ^^cold” phosphor, consisting of a strontium 
sulfide-calcium sulfide base with a lead activator, was 
called Standard I, while the room-temperature phos- 
phor, using the same base but a mixture of rare-earth 
activators, samarium, gadolinium, and europium, was 
Standard II. Although Standard I, emitting in the 
green, was more sensitive than the red-emitting Stand- 
ard II, the difficulty of using a cold phosphor in the 
field caused its further development to lapse in favor 
of the rare-earth phosphor. 

Several factors prevented the immediate use of 
Standard II. Its sensitivity needed to be increased; 
this could be done by shifting the color of emission 
to shorter wavelengths and also by reducing the back- 
ground. The grain size needed to be reduced to permit 
greater resolving power. Improvement in the methods 
of synthesis was neecssary to insure a uniform and 
reproducible phosphor. A detailed knowledge of spec- 
tral properties as well as of the decay of sensitivity 
in storage and in use had to be secured in order to 
determine the best operating conditions. 

Standards III, IV, and V were further develop- 
ments of Standard II ; increasing proportions of stron- 
tium sulfide over calcium sulfide in the basic material 
were used with each type. Standard V using no calci- 
um sulfide at all. This change in base shifted both the 
emission and stimulation spectra to shorter wave- 
lengths (see Figure 1), thus increasing the sensitivity 
for the scotopic eye. Because of this and other improve- 
ments, Standard V showed a 10-fold increase in sen- 
sitivity over Standard III. 

In addition to varying the basic materials, different 
fluxes were tried. Pure strontium sulfide bases at first 
appeared to have the inherent disadvantages of pos- 
sessing a very strong afterglow. However, careful vari- 
ation of fluxes and firing conditions finally led to a 
suppression of the background sufficient to allow the 
use of pure strontium sulfide as a base. This was 
achieved with Standard Y. 

4 2 2 Standard VI 

Simultaneously with the development work on 
Standards III through Y, attempts were made to 
create new types of infrared-sensitive phosphors by 


56 


INFRARED-SENSITIVE PHOSPHORS 


changing the activators. This led to the observation 
that the rare-earth phosphor was one of a large class 
in which two main activators interact to produce a 
sensitivity to stimulation by infrared light which 
neither of the activators would produce if used alone. 
Although interaction of activators was known before, 
it had never been found to produce infrared stimul- 
able phosphors. It was found that an emission spec- 
trum produced by the presence of one activator, the 
dominant, could be sensitized for the stimulating 


as to the ease of excitation with incandescent light. 
And no auxiliary activator was found that gave longer 
wavelength stimulation than samarium in alkaline 
eartli sulfides. Standard VI, the first IRS phosphor 
used in service, was therefore composed of a strontium 
sulfide base like that of Standard V, and a samarium- 
europium activator combination proportioned for max- 
imum sensitivity. 

Several problems arose at this time in regard to the 
mechanical properties of the phosphors. Calcium fluor- 



WAVELEN6TH IN MICRONS 

Figure 1. Spectral characteristics of phosphors with same activators but with different bases. 


action of infrared radiation by the presence of a sec- 
ond auxiliary activator. While the spectrum emitted 
upon stimulation is determined by the dominant, the 
spectral distribution of sensitivity to stimulation is in 
most cases controlled by the auxiliary activator. Fig- 
ure 2 is an illustration of this effect, showing emission 
and stimulation spectra of three phosphors with the 
same base and same auxiliary activator, samarium, 
but with three different dominant activators, cerium, 
manganese, and europium. 

Since it would take many years to investigate all 
the possible activator combinations, a systematic 
search for the best possible pair was out of the ques- 
tion. No dominant activator was in prospect and none 
was found thereafter which could match europium 


ide fluxes produced particularly hard phosphor cake 
whose luminescence was nearly destroyed, however, 
when the cake was ground to a fine powder. Whether 
regenerated by cautious reheating of the powder, or 
regenerated in place on the desired support, no satis- 
factory results on sensitivity were obtained and the 
resolving power of the surfaces was poor. 

Buttons made from the original powdered cake 
under fairly high pressure were reheated beyond the 
softening temperature to cause the grains to coalesce 
and form a smooth surface. Thus, since not as much 
light was lost by scattering from cracks and fissures, 
more radiation could penetrate the phosphor and 
more sensitivity could be gained, as well as a greater 
resolving power. This process introduced some new 




PHOSPHOR DEVELOPMENT 


57 


difficulties. Bubbles or cracks in the surface appeared, 
mainly because of a carbonate content in the phosphor. 
Spots showing a disturbingly bright afterglow were 
also seen ; these turned out to be caused by traces of 
copper and were eliminated by extreme caution in the 



Figure 2. Spectra of three phosphors with same base 
and auxiliary activator. A. Standard VII, dominant ac- 
tivator cerium. B. Dominant activator manganese. 

C. Standard VI, dominant activator europium. 

preparation and handling of the materials. Another 
problem was that the buttons appeared to contract 
during heating, so that the desired precision surfaces 
could not be obtained. Initially, wet giinding with 
alcohol and acid was used to compensate for this, and 
later a dry grinding procedure was devised. 

After a great deal of trial and error, smooth precise 
buttons of a brightness comparable to that of the 
original phosphor cake, or even better, were obtained. 
The resolving power of these grainless surfaces, how- 
ever, was still poor. The reasons for the low resolving 
power seem to be quite complex and ha\'e never been 
saitsfactorily explained. It was assumed that the 
sharpness would be improved if the formation of the 
image was confined to a thinner layer, and appreciable 
gain in resolving power was obtained when this was 
accomplished. This was the last essential change in the 
formulation of Standard VI. 

Threshold sensitivity and resolving powers were the 
main problems in the development of Standard VI. 
The peak of the emission band, still at an unfavorably 
long wavelength, was the best that could be obtained 
with this type of phosphor. The peak of excitation 
around 5,000 A made it possible to excite with a fil- 
tered incandescent light, and the infrared sensitivity 
with peak about 1.02 microns, gradually tapering off 
towards longer wavelengths permitted the use of in- 
frared filters excluding all visible light but at the 
same time making full use of the infrared sensitivity 
of the phosphor. Figure 2 shows the emission and 


stimulation spectra and Figure 3 the excitation spec- 
trum of Standard VI compared to those of some other 
phosphors. 

The infrared sensitivity of Standard VI, as deter- 
mined by threshold measurements, reaches a maximum 
a few minutes after the excitation is removed, due to 
the fast decay of the background and the relatively 
slow decay of the stimulability (see Figure 4). After 
a maximum is reached, the threshold sensitivity de- 
creases slowly, as shown in Figure 5, with the stimu- 
lability obtained after one-half hour being still more 
than half that obtained after one minute. The best 
surfaces showed maximum sensitivity of 10 nautical- 
mile-candles® (defined in Chapter 3). The rate of ex- 
haustion by infrared light is relatively slow, with the 
useful light sum one or two microlambert hours. 

In selecting a suitable combination of filters for 
excitation of Standard Yl, it was necessary to cut out 
radiation below 0.44 micron, which increases the back- 
ground, and above about 0.55 micron, which decreases 
the sensitivity because of a stimulating effect. Al- 
though the quantity of light required for the excitation 



Figure 3. Excitation spectra of several phosphors. 
The curve for B-1 is a very rough approximation. 


of a fully exhausted sample is quite large, the amount 
needed to re-excite a partially exhausted sample is 
small. This is further discussed in Section 4.5. Pre- 
liminary measurements showed that both the inertia 
and time lag of Standard VI were negligible. Me- 


58 


INFRARED-SENSITIVE PHOSPHORS 


clianical properties were satisfactory, although the 
phosphor had to be protected from moist air by en- 
closure in a chamber containing a drying agent, silica 
gel. Quantum efficiency, which should be one in a per- 
fect phosphor, was low in Standard YI since of every 



Figure 4. Comparative decay of background and 
stimulability with time. B, background; S, stimulabil- 
ity. The decay of stimulability and background of B-1 
lies between VI and VII. 

300 incident infrared quanta, only one was converted 
into visible light. This was a great improvement over 
Standard III, however, which has a 1/700 ratio. 

Tentative studies of zinc sulfide phosj^hors with cop- 
per and manganese activators were made, and a few 
tests of the alkali halides conducted. An attempt to 
excite both pure and activated alkali halides by X rays, 
cathode rays, and ultraviolet-spark sources showed 
cases of fair sensitivity to infrared, but none was re- 
garded as promising enough to warrant further in- 
vestigation. The results weie not wholly discouraging 
because the threshold sensitivity of some potassium 
chloride and sodium bromide preparations was of the 
same order of magnitude as that of a fair europium 
samarium sulfide phosphor, although the total light 
sum was very small. Other attempts made with barium 
or magnesium sulfide shoAved little success. 

^2^ Standard VII 

The next phosphor to be successfully developed was 
Standard VII. Since samarium seemed to be by far 
the most satisfactory auxiliary activator in the stron- 
tium sulfide base, giving a strong sensitivity at the 
longest wavelength (tin and bismuth are very effec- 
tive in producing stimulation, but the wavelength is 
too short), it was retained. The choice of the dominant 
activator was made on the basis of numerous experi- 
ments which tried to match most closely the emission 
spectrum with the spectral sensitivity of the scotopic 


eye. Cerium was chosen because of its very suitable 
green emission (Figure 2). The cerium samarium 
phosphor thus produced proved to be about ten times 
more sensitive than Standard VI if excited with ultra- 
violet light and if a sufficiently long period was al- 
lowed between excitation and stimulation (Figure 5). 
More important, it was found that Standard VII was 
also excitable with alpha particles from radium, a 
comparatively rare occurrence in a phosphor of this 
type, becoming then even more sensitive than with 
ultraviolet excitation, and allowing a source to be 
inserted in the same chamber as the phosphor. If suit- 
able filters are provided, daylight excitation is another 
promising possibility. 

It Avas necessary to find a neAv technique for making 
buttons, since that used Avith Standard VI Avas not suit- 
able for the iieAv phosphor, Avhich is chemically much 



Figure 5. Threshold sensitivities of various phosphors 
in Type A metascope. A. Early sample of Standard 

VI. B. Standard VII, ultra\dolet-excited. C. Standard 

VII, radium-excited. D. B-1. 

more sensitive. A method of spraying a crushed pow- 
der on a carbon backing and regenerating in place was 
developed for Standard VII. 

The properties of Standard VII are much more 
favorable than those of Standard VI. Although the 
background of VII is very strong immediately after 
excitation, it decays Avithin a fcAv hours to a suitable 
level, Avhile the infrared sensitivity decays A^ery slowly 
during this time. Figures 4 and 5 shoAv this effect for 
several phosphors, Avith threshold sensitivities defined 
in terms of nautical-mile-candles (nine). The best 
surfaces showed sensitivities of 0.6 nine.® A very large 
light sum is obtained; under infrared illumination 




PHOSPHOR DEVELOPMENT 


59 


the phosphor emits several mierolaml)ert hours before 
its sensitivity drops to one-half of its initial value. 
Quantum efficiency is about 1/300 after ultraviolet 
excitation. Resolving powers obtained with Standard 
VII surfaces are better than those of Standard VI. 
Again, the phosphor is sensitive to humidity and must 
be protected with silica gel. The infrared sensitivity 
of Standard VII is nearly independent of tempera- 
ture from — 70 to 85 C ; at temperatures greater than 
this it drops rapidly. 

A third activator added to cerium and samarium 
showed no improvement, although perhaps the addi- 
tion of lead makes it easier to obtain reproducibility. 

To suit a particular application, an attempt was 
made to obtain fine powders of good sensitivity. The 
addition of magnesium carbonate or magnesium oxide 
combined with a regeneration process yielded fine 
powders that could easily be painted on any surface. 
In such a form the jDhosphor is called Standard Vll-b, 
and sensitivities reached with this powdered phosphor 
excited by daylight are comparable to those found 
with radium-excited molded buttons. 

Selenide Phosphors 

It was suggested that selenides, or a combination 
of sulfides and selenides, would make satisfactory basic 
materials. Two laboratories started work on this prob- 
lem after the development of Standard VI, using the 
activator combination of that phosphor : europium and 
samarium. Emission peaks were found to shift into 
the yellow when a combination sulfoselenide was used, 
but for some time the sensitivity was unsatisfactory 
compared with that of Standard VI. If the mixture 
contained a sufficient predominance of sulfide, the 
sensitivity was about that of Standard VI, but the 
red color of emission was also approximately the same, 
and there was no net gain in sensitivity. Further 
work, however, showed that a phosphor of remarkably 
high sensitivity (B-1) could be made when the pro- 
portion of selenide was greater than 80 per cent, with 
a favorable yellow emission. 

Although many other activator pairs were tried, 
none came even near to the high infrared sensitivity 
of the europium-samarium combination. Bismuth was 
second to samarium as an auxiliary, showing less shift 
to shorter wavelengths in a sulfoselenide than in a pure 
sulfide base. In a bismuth-samarium combination, it 
appears that bismuth largely determines the stimula- 
tion in both sulfide and selenide bases. 

An investigation of a group of bases containing the 


four ions of calcium, strontium, selenium, and sul- 
fur was carried out, confirming the hypothesis that 
the combination containing mainly strontium selenide 
is the most favorable. 

As in the transition from Standard II to Standard 
VI, the transition from VI to the selenide phosphor B-1 
involves a change of base and achieves an advantage of 
a further shift of emission toward shorter wavelength 
( Figure 1 ) , accompanied by a shift of the stimulation 



-2 0 2 4 


LOO IN TIME X INTENSITY OF EXHAUSTING LIGHT 

Figure 6. Phosphor decay with exhaustion. Dots, 
experimental points; solid line, calculated curve. 

and excitation bands to shorter wavelength in an un- 
favorable direction, though not seriously so. The B-1 
phosphor, like Standard VII, has a rather high back- 
ground which becomes negligible after about one-half 
hour, more slowly than Standard VI but more quick- 
ly than Standard VII. The sensitivity is stored for 
periods of days and the useful light sum is very 
large (Figure 5). Best surfaces showed sensitivities of 
1.5 nautical-mile-candle. 

Figure 6 is a graph of the exhaustion curves, or the 
decrease of sensitivity with the amount of infrared 
stimulation of the practically important standard 
phosphors. The fact that in both Standard VII and 
B-1 useful light sums of several microlambert hours 
are available is one of the most important features of 


60 


INFRARED-SENSITIVE PHOSPHORS 


the infrared phosphor development. These large light 
sums permit long-continued use with intensities oc- 
curring in signaling; even accidental strong over- 
exposures will affect the performance only in excep- 
tional cases. 

Excitation of B-1 is effected by a broad band in the 
visible and very near the ultraviolet. It extends from 
about 4,200 to 5,400 A and is practically zero at 3,800 
and 6,200 A. The total amount of incandescent light 
needed to produce full excitation is somewhat larger 
than that for Standard VI, but the danger of “over- 
exposure” by too high exciting intensities that exists 
with A"I is negligible in B-1. The stimulation of B-1 
has a peak at 0.93 micron with a half-width of about 
0.2 micron, and the emission peak at 0.57 micron is 
in a favorable position. If equal numbers of emitted 
quanta are considered, an evaluation of the emission 
bands in terms of scotopic vision yields relative values 
of 1, 10, and 25 for Standard VI, B-1, and Standard 
VII, respectively. Quantum efficiency is roughly the 
same as in Standards VI and A^II. 

B-1 has one serious disadvantage. Preparing, han- 
dling, and protecting the selenides from the atmos- 
phere is much more difficult than with the sulfides; 
the finished surfaces must be protected with a ceresin 
wax coating. This may contribute to the relatively low 
resolving power of B-1. In addition, there is a con- 
siderable inertia of the infrared response at low in- 
frared intensities. 

The threshold sensitivity reached with the B-1 phos- 
phors was much higher than that obtained with Stand- 
ard AT, but did not quite reach that obtained with 
Standard VH. 

2 5 Zinc Sulfide Phosphors 

An extensive search for stimiilable phosphors sen- 
sitive to longer infrared wavelengths yielded no useful 
results as far as field use was concerned. Even if the 
stimulation peak was pushed to the desired longer 
wavelengths, the sensitivity at that wavelength was 
still less than that of Standard VII. 

Preliminary observations of several thousand zinc 
sulfide phosphors showed that the most sensitive ones 
were those activated with copper and manganese. An 
infrared-sensitive phosphor with copper and terbium 
as activators has a red stimulated emission; in both 
the copper-terbium and copper-manganese types the 
color of the afterglow is different from that of the 
stimulated emission, and the latter is less favorable 
to the scotopic eye. 


However, more promising results were obtained 
when a large amount (as much as 6 per cent) of the 
single activator, lead, was used. The emission is blue- 
green and very favorable for scotopic vision. Second 
to lead, lanthanum and gadolinium produced good 
sensitivity. In no case did the sensitivities of these zinc 
sulfides reach that of Standard VI. If a very small 
amount of another activator, copper, manganese, lan- 
thanum, or gadolinium, was added as an auxiliary to 



Figure 7. Emission and stimulation spectra of zinc 
sulfide copper lead phosphor. 


the dominant lead, better phosphors were obtained. 
With the best zinc sulfide materials, careful prepara- 
tion, and appropriate heating cycles, an optimum of 
sensitivity was found with 4 to 7 per cent lead sulfate 
and 1 to 10 micrograms of copper per gram of zinc 
sulfide. Figure 7 shows the emission and stimulation 
spectra of a typical zinc sulfide lead phosphor. 

The optimal copper concentration depends on the 
use to which the phosphor is put ; if observation short- 
ly after excitation is desired, the lower copper concen- 
trations are more favorable. Highest threshold sen- 
sitivities were obtained with concentrations of 3-10 
ppm of copper and waiting periods of several hours 
after excitation, to allow the very bright background 
to decay. Under these conditions thresholds compar- 
able to those of Standard VI were obtained, so that 
these phosphors were suggested for use in practical 
tests. Results of these tests were very disappointing 
and by no means confirmed the laboratory data. The 
discrepancy was explained by the fact that in the field 
a moving instrument was used, while in the laboratory 




CHEMICAL AND MECHANICAL PREPARATION OF PHOSPHORS 


61 


everything was stationary. These phosphors show a 
strong inertia to stimulation which naturally affects 
the field tests more unfavorably. This effect becomes 
more and more pronounced the longer the waiting 
period after excitation. 

These phosphors were so near to practical useful- 
ness that more complicated procedures were tried for 
their operation in the field. One method was to heat 
the phosphors immediately after excitation so as to 
remove the background in a short time, but this pro- 
duced the same inertia effect as waiting. Operating 
the phosphor at low temperature did not yield any 
particular improvement. An important experiment 
was made that showed promise : If a sample is excited 
at the temperature of liquid nitrogen and stimulated 
at that temperature with radiation beyond 1.0 micron 
and then warmed to room temperature, the sample 
shows no appreciable background while its sensitivity 
is as high and its light sum as favorable as that ob- 
tained with normal excitation. However, this has the 
same obvious objection for field use as that which 
applied to Standard I. 

The stimulation spectra in zinc sulfide phosphors 
are very different from those in the strontium sulfide 
or selenide groups. All zinc sulfide phosphors seem to 
show infrared sensitivity with one stimulation peak 
between 0.6 and 0.8 micron, and many of them show 
a band with a peak at 1.32 microns, which is the one 
desired. It has been found that numerous manganese 
phosphors show only the first of these bands and that 
the addition of cadmium sulfide decreases the sensitiv- 
ity of the long wavelength band. Magnesium chloride 
and lithium fluoride fluxes used with heating tempera- 
tures less than 1000 C also suppress long wave- 
length stimulation while zinc orthophosphate and high 
muffling temperatures develop it. In addition, there 
are some indications that the long wavelength band 
has something to do with the presence of copper. 

At low temperatures, the stimulation spectrum of 
zinc sulfide phosphors seems to change considerably; 
the two bands are replaced by one broad band with 
the peak between the two room-temperature peaks but 
extending less far into the infrared than does the 
separate 1.32-micron band. 

Other interesting effects have been noted with the 
zinc sulfide phosphors. While the spontaneous emis- 
sion or afterglow of the zinc sulfide copper-lead phos- 
phors can be represented by a straight line in a log- 
log plot, the stimulated emission yields a straight line 
on a semilog plot, indicating an exponenfial mono- 
molecular decay. The spontaneous emission is mainly 


determined by the copper, and the stimulated emission 
by the lead. 

Simple theoretical considerations indicate the pos- 
sibility of obtaining response at much longer wave- 
lengths by reducing the temperature ; therefore a com- 
prehensive survey was made with several hundreds of 
phosphors. While peaks of infrared response were 
found up to 1.8 microns and appreciable tails to at 
least 2.5 microns, the absolute sensitivities were so 
low that even the most efficient cold phosphors, par- 
ticularly the selenides, were less efficient up to 1.4 
microns than any good standard infrared phosphor. 
Beyond this limit, the sensitivities may be better with 
the cold phosphors, but they are still far too low for 
military use. 

A search for new bases achieved little success. Lan- 
thanum oxy sulfide phosphors showed infrared sen- 
sitivity with lead-europium and indium-lead pairs; 
zinc germanate activated by the addition of tin and 
manganese oxides showed quite high stimulability but 
very bright backgrounds. Zinc orthophosphate phos- 
phors with lead, and magnesium and barium silicate 
phosphors with europium and samarium were also 
investigated, but none of these seemed promising 
enough to warrant further development at the time. 

4 3 CHEMICAL AND MECHANICAL 
PREPARATION OF PHOSPHORS 

Purity of Materials 

If phosphors of long duration are desired, and par- 
ticularly if attention is given to details of their light 
storage (response to infrared radiation), the require- 
ments on the purity of the materials which make up 
the phosphors are even higher than those for fluores- 
cent and cathode-luminescent materials. 

Thus far, the only phosphors of practical value for 
infrared detection have been prepared with alkaline- 
earth sulfides and selenides as bases. Nearly the only 
information on the synthesis of highly sensitive phos- 
phors of this type is that contained in the papers of 
Lenard and his pupils. In these papers, the prescrip- 
tions given for the synthesis of a particular phosphor 
very often contradict one another and often are not 
repeatable by another group. Nor are some general 
statements made by the Lenard group to be trusted 
as final and absolute. For example, there is the state- 
ment that in sulfide phosphors the presence of sulfate 
in quantities as high as 80 per cent is not significant. 
This was shown to be false before the present work 




62 


INFRARED-SENSITIVE PHOSPHORS 


began and has become more evident as the work con- 
tinued. Cationic as well as anionic impurities are of 
the greatest importance, as they may act as additional 
activators and change the whole characteristics of the 
phosphor. 

By proper purification and handling, it is possible 
to free a phosjDhor material from impurities to a 
higher degree than even spectroscopic methods can 
discern. This is necessary because anionic impurities 
may influence results in quantities as small as 100 
ppm, and cationic impurities in quantities of 10~^ 
ppm. Details of purification of the materials used are 
not given here, but can be found in references 8 and 9 
in the bibliography. 

^ ^ ^ Influence of Fluxes 

One important purpose of a flux in a phosphor is to 
allow the basic material to flow into a definite matrix 
in which the activators are embedded in a certain crys- 
tal structure. It has been found, however, that the 
chemical reactions that take place between the base 
and flux can also be of the greatest importance. 

Previous mention has already been made about the 
influence of flux and heating cycle on the background. 
Further advances were made at the very beginning of 
the investigation of the selenide phosphors. The first 
preparations used a selenide base that had been ob- 
tained from the reduction of selenite, and fluxed with 
strontium sulfate and calcium fluoride. It was found 
that, when the sulfate in the flux was replaced by sul- 
flte, better results were obtained. The sulfite appar- 
ently formed some sulfide as well as an oxygen ion and 
free selenium. The selenium was not desirable, but 
evaporated at the fluxing temperatures used, while the 
presence of strontium oxide and strontium sulflde in 
small amounts materially increased the sensitivity of 
the phosphor. It was finally found that the addition 
of both sulfide and sulfate to the flux produced the 
necessary ions and the best resulting matrix. Addition 
of the sulfate also makes the material adhere more 
strongly to a graphite base and increases the absorp- 
tivity of the phosphor fllm. 

4.3.3 Preparation of Standard Phosphors 

Standaed VI 

The first infrared-sensitive phosphor to be used ex- 
tensively was prepared by methods much cruder than 
those used for the later production of Standards VII 
and B-1. 


Strontium carbonate is slurried with water, dosed 
with solutions of samarium and europium to yield 
concentrations of approximately 200 ppm of each 
gram of strontium carbonate, and dried. The carbo- 
nate is mixed with % of its weight of distilled sul- 
fur, placed in a 120-150 ml covered platinum dish and 
heated in a gas furnace at 1200 to 1250 C. This charge 
is heated for 12 to 15 minutes depending on its size, 
with a stream of hydrogen sulfide or nitrogen laden 
with carbon disulfide vapor continually passing into 
the furnace. 

To flux, a mixture of 100 parts of the now activated 
strontium sulfide, 40 parts distilled sulfur, and 6 parts 
of chemically pure calcium fluoride, is fired as before 
at 1000 C for 30 minutes. The phosphor product is 
then ground to pass a 60-mesh silk screen and stored 
until needed. 

A button is made by first grinding the fluxed phos- 
phor to pass a 200-mesh screen. Then the requisite 
amount of material is pressed into a capsule to form a 
cone-shaped heap, and the capsule is tapped to dis- 
tribute the powder in a symmetrical mound which is 
molded to the desired shape by an iron mold. This is 
placed in a cold gas furnace and heated to 990 C. The 
furnace is allowed to cool to 250 C and the capsules 
removed; a stream of nitrogen is passed through the 
furnace during both heating and cooling. Buttons so 
formed are further processed by lapping to precise 
shape by means of a ground-glass mold and dilute acid. 
An extension of this method made use of mechanical 
grinding on a lathe with an Alundum grinding wheel 
of proper curvature. 

This method of forming buttons has been super- 
seded by the high-pressure technique used with B-1 
and Standard VII, and Anally by the use of regener- 
ated fine powders of high sensitivity painted onto the 
desired surfaces. 

B-1 

One thousand grams of strontium selenide, 75 grams 
of activated calcium fluoride, 75 grams of strontium 
sulfate, and 50 grams of strontium sulfide are screened 
through 150-mesh silk, mixed in a mortar and milled 
for 2 hours in a ball mill. This mixture is then stored 
in paraffin-stoppered bottles until ready for use. 

The B-1 powder mixture is molded in a Loomis 
hydraulic press at 16,000 to 20,000 pounds. This mold 
is made of a pressure-molded slab of ignited chem- 
ically pure magnesia, so constructed that when fired 
the button conforms to the dimensions required for 
use. The molded button is finally fired in a quartz 




METHODS OF MEASURING PHOSPHOR CHARACTERISTICS 


63 


tube in oxygen-free nitrogen for exactly 10 minutes. 
Protection of the button is achieved by dipping in 
ceresin wax. 

Standard VII 

Two hundred and eighty grams of strontium sulfide, 
28 grams of chemically pure magnesia (ignited in air 
at 1050 C), 13.4 grams of activated lithium fluoride, 
and 28.5 grams of strontium sulfate are screened 
through 150-mesh silk, mixed in a mortar and milled 
for an hour and a half. The powder mixture is placed 
in covered platinum boats of 80 to 85 grams capacity 
and fired in pure nitrogen at 1050 C for 30 minutes; 
cooling is also accomplished in the nitrogen atmos- 
phere. The product, after inspection for abnormalities, 
is ground in a porcelain mortar to about 16 mesh and 
then milled for 2 hours. After screening through a 
300-mesh silk, the powder is bottled and stored in a 
desiccator. 

A thin coating of Standard VII powder is applied 
to a graphite button (grade C-15, National Carbon 
Company) of the proper radius of curvature, by first 
suspending the phosphor powder in a solution of 
methyl methacrylate in ethylene chloride and then 
paint-spraying the suspension onto the graphite but- 
tons. After drying, the buttons are fired in an air- 
free nitrogen atmosphere; they are placed on steel 
trays and inserted into a quartz muffle, heated to 
860 C and kept at this temperature for 20 minutes. 
They are cooled under nitrogen and then inspected 
for faults. 

44 METHODS OF MEASURING 

PHOSPHOR CHARACTERISTICS 

In order to secure complete information about a 
particular phosphor, many different quantities must 
be measured. Of these the most important are (1) 
the stimulability ; (2) the infrared sensitivity, de- 
fined as the reciprocal of the infrared illumination re- 
quired to produce a just detectable bright spot on the 
phosphor, or threshold sensitivity; (3) the background 
or afterglow; (4) the spectral characteristics; (5) the 
resolving power; and (6) the inertia and time lag. 

^ Qualitative Observations 

For nearly every phosphor the most important meas- 
urement is that of stimulability. Such a test gives at 
once a good idea of the value of the phosphor for in- 
frared detection, and in many cases a direct measure 
of its infrared sensitivity. In most cases a qualitative 


inspection under an incandescent lamp with various 
infrared filters was made, varying the intensity of in- 
frared by the distance from the lamp, and using vari- 
ous waiting times after excitation. When enough in- 
tensity and duration of infrared illumination was 
used, indications of the brightness and behavior of 
the afterglow and of the useful light sum were also 
obtained. These qualitative tests were usually made in 
conjunction with some standard; very frequently a 
whole set of preparations, differing in one well-de- 
fined respect such as flux, activator concentration, 
or heating cycle, were observed simultaneously. Crude 
threshold comparisons were made with a weak infrared 
illumination if preparations of strongly differing 
background were compared. 

If more than usual importance was attributed to 
some preparation and if its reproducibility was re- 
garded as well-defined, its infrared stimulability was 
quantitatively measured, as later described. 

In many cases, it was necessary to obtain a general 
idea of the spectral distributions of emission, excita- 
tion, and stimulation of a phosphor as well as of the 
quenching spectrum. Emission spectra could be ob- 
served either visually or photographically with a high- 
aperture spectroscope. This was particularly useful 
for the recognition of different coexistent bands, or 
line emission superimposed on broad emission bands. 

Excitation spectra are very important since knowl- 
edge of these must be obtained before proper excita- 
tion of the phosphor for infrared sensitivity tests can 
be accomplished. The standard arrangement for this 
purpose consisted of a spectroscopic device, projecting 
the spectrum of a suitable light source (carbon arc, 
spark, mercury arc) on a phosphor spread in the hori- 
zontal plane of the spectrum. An infrared light source 
was frequently arranged above the phosphor and in 
some cases a small direct-vision spectroscope was used 
so that the emission spectra |)roduced by different ex- 
citing wavelengths could be observed. Experiments 
made with the phosphor spread on a heating tray, so 
as to permit observation at different temperatures, 
gave perhaps the most comprehensive information 
available with qualitative observations. 

Stimulation spectra and quenching in the visible 
and infrared region were also observed by means of a 
spectrum projected on a phosphor. Although infrared 
stimulation was desired throughout this work the 
stimulation bands were by no means confined to this 
region. In order to make the extinguishing effects 
(stimulation or quenching) of various wavelengths 
plain, an infrared light source was arranged so as to 


iffiy'i'iilij'i'l'if'" 


64 


INFRARED SENSITIVE PHOSPHORS 


make possible an even flooding of the phosphor with 
infrared light after an exposure to the spectrum. This 
is in many cases the most sensitive means for detecting 
weak stimulation or quenching. 

Measurements 

Threshold 

Much time was spent in the development of a 
method of measuring threshold which would give 


exhaustion, and absorption spectra, and, with slight 
modifications, for nearly any determination that re- 
quired high precision or monochromatic light. Many 
of these measurements had to be carried out at very 
low intensity levels, since it was necessary to maintain 
the phosphor in very nearly the same state of excita- 
tion throughout a series of measurements. In the de- 
termination of a stimulation spectrum, for example, 
it is permissible to measure the whole spectrum with 


MIRROR 



Figure 8. Light source for threshold measurements. P, metal plate with set of calibrated pinholes; L, 10-watt lamp; 
G, ground glass; A, aperture; *8, movable meter stick. 


reasonably consistent results and would agree with 
tests made under the conditions of practical use. None 
of the arrangements developed was considered by all 
workers as wholly satisfactory. Much of the difficulty 
was caused by the fact that a device with an aperture 
similar to that of the Kellner-Schmidt system (see 
Chapter 3) was desirable. In such a device the focus- 
ing is very critical, and the thresholds obtained are 
sensitive to the focusing. Since threshold determina- 
tions are necessary for all phosphors which are serious- 
ly considered for practical use, and are therefore ob- 
tainable in button form, the use of a metascope (Chap- 
ter 3) and a properly illuminated pinhole is very 
helpful. This device is shown schematically in Fig- 
ure 8. 

Spectral Measurements 

Figure 9 shows the method that was used for the 
determination of excitation and stimulation spectra, 


one single excitation only if the first measurement can 
be repeated at the end of the series with the same re- 
sult as at the beginning. Another reason for the need 
for highest sensitivity is the desire to make the meas- 
urements at the intensity level at which threshold 
observations take place. Thus the sensitivity of the 
measuring arrangement should be such that a just 
detectable brightness of a fairly small phosphor sample 
can be measured without great difficulty. 

The desired sensitivity was achieved by using a 
photocell in conjunction with the FP54 electrometer 
tube amplifier. A current sensitivity of 10“^^ ampere 
was used in extreme cases with fair stability. High- 
aperture mirrors collected the light from the phosphor 
and cast it on the photocell; in the most favorable 
arrangements, more than 50 per cent of the light flux 
emitted by the phosphor was collected on the photo- 
sensitive surface. By these means, the emission of a 
phosphor of any color could be measured conveniently, 




METHODS OF MEASURING PHOSPHOR CHARACTERISTICS 


65 


although it might be absolutely invisible to a well- 
dark-adapted observer. 

With the exception of extreme cases, sufficient sen- 
sitivity could be reached by the use of multiplier 
phototubes. For measurements in the red and in- 
frared regions, cesium phototubes were used with a 
Vance amplifier. 

Direct absorption measurements on the phosphor 
powders by determination of their tiansmissions are 
difficult to carry out and not simple in their signifi- 
cance. Measurements were therefore made of diffuse 
reflectance, permitting the calculation of the ratio of 
the coefficients of absorption and scattering. Within 
the w^avelength ranges concerned, the scattering is 
regarded as constant, and thus the ratio is proportional 
to the absorption coefficient. 

Monochromator-photocell arrangements were again 
used for emission spectra measurements, with pro- 
vision made for alternating excitation and stimulation 
while measuring the stimulated emissions. Photo- 


11 esolving Power 

For practical reasons it was necessary to make some 
kind of resolving power tests. Initial experiences 
showed that a really thorough investigation of this 
problem would require very much effort and man- 
power, for simple procedures proved to be extremely 
unreliable. 

A statistical method was used at one of the labora- 
tories with good success. The objects to be resolved 
were two pinholes illuminated in a manner similar 
to that used for the pinhole in the threshold-measuring 
apparatus. A set of small brass plates, each with a 
pair of small pinholes, was constructed with the dis- 
tances between holes chosen so as to make the cor- 
responding values of the angular separation go from 
10 to 30 minutes of an arc in steps of 2 minutes. To 
this set was added a few similar plates which had only 
one pinhole. All of the pinholes in the plates were of 
the same size. To make a measurement, the plates 
were shuffled to give a random order and then placed 




Figure 9. Monochromator and FP54 setup. 


graphic methods for quantitative determination of 
emission spectra with the usual methods of photo- 
graphic spectrophotometry were also used for this 
purpose. With both of these methods, the emission 
spectra of background as well as those of stimulated 
emission were investigated, the photographic method 
proving superior for weakest emissions. 


in position and observed (with the placing of the plates 
done by someone other than the observer whenever 
possible). The observer then described the image as 
single or double as it appeared to him. If he called all 
plates above a certain separation double and all bel- 
low it single, the measurement was considered valid 
and the separation recorded as the limit of resolution. 




66 


INFRARED-SENSITIVE PHOSPHORS 


A photographic method was used to determine 
graininess as well as resolving power. An infrared 
image of an evenly illuminated slit 1 millimeter wide 
and 100 millimeters long was projected on a phosphor 
through an infrared-corrected microscope objective 
by means of a vertical illuminator. The visible light 
emitted by the phosphor was projected by the same 
microscope onto a photographic plate sensitive to the 
phosphor emission but not to infrared. This image, 
which should be geometrically identical with that of 
the original slit, was quite diffuse and showed the 
graininess of the early phosphors very well. In order 
to obtain a measure of the sharpness independent of 
the exposure time, a steep neutral wedge was placed 
in contact with the photographic plate with its max- 
imum gradient parallel to the slit image. With weak 
and short infrared exposures and re-excitation be- 
tween exposures, photographs were obtained showing 
an approximately triangular isodensity contour. The 
steepness of the triangle is a measure of the sharp- 
ness or resolving power, and allows quite small differ- 
ences in resolving power to be detected. Unfortunately, 
the correlation of these measurements with practical 
resolving powers has not been investigated. 

Sensitivity 

The gradual decrease of sensitivity under continu- 
ous infrared illumination has been investigated for 
both its practical and theoretical importance. Al- 
though the obvious subject to investigate is the 
brightness as a function of time under constant in- 
frared illumination, such a measurement is difficult 
because of the very large ranges of sensitivities to be 
covered with most phosphors if anything approaching 
full exhaustion is to be reached. Since it was thus de- 
sirable to change the infrared intensity during the 
progress of exhaustion, an investigation was carried 
out that showed that a reciprocity law for the effect of 
time and intensity of infrared radiation holds, making 
possible the piecing together of sections taken at dif- 
ferent infrared intensities into a single exhaustion 
curve. 

In these measurements the infrared emission was 
obtained and plotted as a function of time represent- 
ing the law of exhaustion. For theoretical reasons, it 
was also desirable to obtain the plot of infrared sensi- 
tivity against the light sum or against absorption. If 
the exhaustion is carried out properly, this plot can 
easily be obtained from experimental data. In some 
of these experiments, the effective intensity was in- 
creased gradually in a known manner ; and sometimes 


a simultaneous measurement of infrared reflectivity 
of the phosphor and the visible emission was carried 
out while the exhaustion was proceeding. 

Along with these quantitative investigations of the 
processes of stimulation, a quantitative study was made 
on the release of stored light by thermal energy. The 
measurement of emitted brightness during a gradual 
increase in temperature was carried out for a number 
of cases, with automatic recording of the brightness 
and temperature. Spectral sensitivity of the photocell 
used for brightness measurement was flattened to such 
an extent that it could be regarded as an energy- 
measuring device. For the comparison of stimulated 
and thermoluminescent emission, the two experiments 
were carried out on the same apparatus. 

Time Lag 

Delays in the visible response of an IRS phosphor 
to an abrupt beginning or ending of a period of in- 
frared illumination are closely connected, and if they 
are too long they become an important setback to the 
practical use of the phosphor. These delays are dis- 
astrous in the zinc sulfide copper lead phosphors, quite 
pronounced in B-1, and still observable in Standard 
YII. Measurements on Standard YII have been made 
by four methods: (1) the observation of the flicker 
limit obtainable; (2) an oscillographic investigation; 
(3) a series of phosphoroscopic investigations; (4) 
direct decay measurements of the slow component of 
the time lag. With B-1 some direct measurements of 
the inertia were carried out. The complexity of this 
part of the subject is so great that no attempt will 
be made to describe it further here; details may be 
obtained from the contractors’ reports listed in the 
bibliography. 

4 5 GENERAL THEORY AND ITS APPLICA- 
TION TO PRACTICAL PHOSPHORS 

Although the theory of IRS phosphors is still in 
its infancy, it is necessary for further development to 
understand at least the simplest parts of the proposed 
theories. This section is included for the purpose of 
making clear the fundamental reactions that are re- 
sponsible for the characteristics of phosphors, as far 
as they are known at present. 

Exhaustion Curves 

From a theoretical point of view, probably no single 
piece of information on the IRS phosphors is as in- 
teresting as are the exhaustion curves. Since it had 


GENERAL THEORY AND ITS APPLICATION TO PRACTICAL PHOSPHORS 


67 


been assumed that the exhaustion was a bimolecular 
process, it was thought that a function representing 
the superposition of several second-order decays could 
be constructed to follow the phosphor exhaustion (see 
the solid curves of Figure 6). Although this succeeded 



0 as 1.0 

FRACTION OF INITIAL LIGHT-SUM 


Figure 10. Stimulability against fraction of initial 
light sum. 

fairly well with Standard VII, the exhaustion of B-1 
could not be fitted over any considerable range. Ee- 
cently, after the expiration of NDRC contracts, the 
B-1 curve was fitted with a single second-order decay 
using a correction for absorption in the phosphor. 

In addition to plotting the brightness against the 
time of exhaustion, it is useful to represent sensitiv- 
ity as a function of the light sum still contained in 
the phosphor (Figure 10). The general shape of an 
exhaustion curve of this sort, beginning with strong, 
quickly decaying emission and tapering off into an 
apparently endless tail with very slow diminution of 
the still appreciable brightness, suggests immediately 
that excited states of very different sensitivities must 
be present initially in a fully excited phosphor. Here 
the brightness represents the rate of return to the 
ground state while the light sum represents the re- 
maining number of useful excited states. 

Curves like that of Figure 10 may also be used to 
make an estimate of the order of magnitude of the 
absolute sensitivities of the few highly sensitive states 
which, upon stimulation, return first to the ground 
state. When calculated in relation to the number of 
infrared quanta absorbed, the quantum efficiency for 
the most sensitive states turns out to be about one- 
fifth in Standard VI and very nearly unity in Stand- 
ard VII. The relatively low practical efficiency of these 
two phosphors is mainly due to the still large reflect- 


ance of the excited phosphors in the region of their 
stimulation maximum, and to the presence of a rela- 
tively large number of excited states of low sensitiv- 
ities compared to a small number of highly sensitive 
ones. Standard VII, which absorbs only about 4 per 
cent of the incident infrared, is mainly affected by 
the first characteristic while Standard VI is affected 
mainly by the second. 

A third method of plotting the exhaustion is to 
represent the number of excited states (or stimula- 
bility) as a function of the absorption coefficient of 
the stimulation peak. This is shown in Figure 11. 
If the same sensitivity is produced by partial excita- 
tion and by complete excitation with partial exhaus- 
tion, the absorption and light sum of the two states 
may be very different. Conversely, the same light sum 
or absorption may be accompanied by very different 
sensitivities. This is easily explained by again assum- 
ing the coexistence of excited states of different sen- 
sitivities. Part of the difference should be due to the 
finite depth and the absorption of the phosphor layers. 

Another approach to the problem of efficiencies 
was made by correlating the constants in the second- 
order equation, fitting the decay curve of Standard 
VII with the number of excited states and the intrinsic 



0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 


RELATIVE VALUES OF ABSORPTION COEFFICIENT 

Figure 11. Sensitivity against absorption during 
excitation and exhaustion. 

efficiency of the most sensitive states. The results 
found by using different cerium-samarium phosphors 
of various samarium content showed that while the 
efficiency of such a group lemained nearly constant, 
the number of excited electrons varied considerably. 




68 


INFRARED-SENSITIVE PHOSPHORS 


Excitation after Partial Exhaustion 

Tlie amount of light needed to excite a fully ex- 
hausted Standard VI or B-1 phosphor is considerable, 
but if the phosphor has once been fully excited and 
has lost only part of its sensitivity by stimulation or 
by spontaneous decay, the restoration of full sensitiv- 
ity requires very much less exciting light. For ex- 
ample, if 40 per cent of the full sensitivity of Stand- 



O 400 800 1200 1600 2000 2400 2800 3200 3600 4000 

EXCITING LIGHT IN FOOT-CANOLES PER MINUTE (MEASURED WITHOUT FILTER) 

Figure 12. Sensitivity in per cent of saturation. 

ard Y1 is lost by exhaustion, only a few per cent of 
the amount of light originally necessary is required 
to bring it back to full sensitivity. See Figure 12. As 
in the preceding paragraphs, the explanation of this 
is probably that excitation of a fully exhausted phos- 
phor produces many excited states. Of these, exhaus- 
tion removes selectively only the most sensitive states, 
so that a considerable drop in sensitivity means only 
a small reduction in the total number of excited states. 
This loss is easily replaced by re-excitation, since now 
the only states that are free are those of high sensi- 
tivity. Although the re-excitation phenomenon is not 
pronounced with ultraviolet excitation of Standard 
VII, it is quite distinct with radium excitation. 

4.5.3 Thermal and Stimulated Emission 

Figure 13 shows typical examples of glow curves, 
or the gradual release of light by an excited phosphor 
as its temperature is raised. Lenard had previously 
stated a rule that the total light sum emitted on stim- 
ulation was always small compared to the amount 
emitted on heating. However, in phosphors with high 
infrared sensitivity, cases are found where the oppo- 
site holds true, such as with Standard VII, which 
emits much more light upon stimulation than upon 
heating, and with B-1, where the ratio is as large as 
100. In Standard VII, the color of the light stimu- 
lated by infrared is distinctly different from that 
emitted by heat; the first is probably due to cerium 
and the second to samarium bands. At first this was 
thought to have something to do with the light sum 


discrepancy, but the same anomalous effect was found 
in Standard VI and in manganese-samarium phos- 
phors where the two colors are equal. 

Saturation 

After the initial stages of excitation, when the sen- 
sitivity of the phosphor increases, usually with the 
square of the amount of exciting light, the sensitivity 
reaches an equilibrium saturation value. The rate at 
which this process takes place and the saturation value 
obtained depend on the intensity of the exciting light 
and its wavelength, and these two factors are differ- 
ent for different phosphors. In many cases, there is 
little dependence if the intensity is so high that the 
building up of sensitivity is fast compared with the 
spontaneous decay, and the saturation value reached 
in such cases is not determined by an equilibrium 
between the decay and the rate of excitation. Thus 
the saturation must be due in part to an exhausting 
effect of the exciting light itself, especially important 
in Standard VI, where too high exciting intensities 
must not be used if greatest sensitivity is desired. 

^ ^ ^ Decay Phenomena 

There is a distinct relationship between the stimu- 
lability o-, the infrared threshold sensitivity S, and 
the background G, of a phosphor. Up to a limiting 



TIME IN MINUTES 

Figure 13. Glow curves for Standards VI and VII. 


background the sensitivity is not influenced by 
the background. Any afterglow less than this will 
not appreciably affect the sensitivity and S, in appro- 
priate units, will be the same as o-; at the same time 
this faint background may be of great help in locat- 
ing the proper eyepoint in an instrument. With a 
stronger background, the relation can be fairly well 
represented by the following equation. 



or 

log .S' = log (T — ?i log G n log G*, 




GENERAL THEORY AND ITS APPLICATION TO PRACTICAL PHOSPHORS 


69 


For practical purposes, G* is of the order of 10 micro- 
niillilamberts and n is roughly 0.5. Thus, if the stim- 
ulability and afterglow are known as functions of time, 
the sensitivity at any time can be calculated without 
the need of making threshold measurements. 

If the decay of stiniulability is very slow compared 
to that of the background, the threshold sensitivity 
will increase until the background has dropped below 
the limiting value G* and then will decrease slowly 
as the stimulability drops. For most good standard 
phosphors, this is a fair approximation and thus a 
simple measurement of stiniulability will give a meas- 
ure of the threshold. Standard I, however, with a 
single activator, acts in quite another manner and 
seems to be typical of most singly activated phos- 
phors. Its stimulability, remarkably high immediately 
after excitation, drops seriously with the slowly decay- 
ing strong background and becomes very low when 
G is less than 

A reasonably general formulation may be given by 
using a Becquerel-type formula for phosphor decay. 
This yields simplified expressions sufficient for values 
of t that are not too small : 

log (T = Cl — log t, 

log G = C 2 — Wg log t. 

In most cases, is very near to one while Wi is con- 
siderably smaller, and the sensitivity formula can be 
simplified. 

log S = C { rim 2 — w^i) log t. 

Thus the condition necessary for improvement of 
threshold on waiting after the end of excitation is 
simply that m 2 must be greater than m-^. Table 1 
shows values of and mg for some important cases; 


Table 1. Decay constants of several phosphors. 



mi 

m 2 

Standard VI 

0.28 

1.14 

Standard VII 

0.10 

1.10 

Zinc sulfide lead 

0.26 

1.18 

Strontium sulfide copper 

0.20 

0.96 


in all of them the above condition is met by a con- 
siderable margin. The physical meaning of the differ- 
ence between mg and mj depends upon whether there 
exists a connection between thermal instability and 
infrared sensitivity of the excited states. If the two 
quantities are equal there is complete parallelism, 
and if their difference is 1 there is complete inde- 
pendence. In the first case, waiting after excitation 
would be of no value since reduction of the thermal 
background would reduce the stimulability propor- 
tionately. In the second case, the thermally most un- 
stable states are lost by waiting, but if they are only 
a small fraction of all excited states, the stimulability 
is little impaired and the threshold improved. 

Knowledge of these decay properties is of great 
importance for developing methods of operating the 
phosphors. The very slow spontaneous decay of sen- 
sitivity in Standard VII, for example, makes it pos- 
sible to use very slow excitation from a weak radium 
preparation ; the same good storage permits use 
throughout a whole night without re-excitation. Day- 
light excitation (a weak source) is not sufficient to 
excite the fast-decaying Standard VI. The relatively 
small difference in the decay constants of the stron- 
tium sulfoselenide copper phosphor shown in Table 1 
is enough to make this phosphor, which has a prom- 
isingly long wavelength stimulation at 1.4 microns, 
show little threshold sensitivity at any time. 


‘"TlLSlllWiiMkftK. 


Chapter 5 


SURVEY OF INFRARED SOURCES 


By George E. Meese^ 


51 INTRODUCTION 

T his chapter has been prepared to record the de- 
velopment and/or application of the various in- 
frared sources in projects of Section 16.5, NDRC. 

Information is given on three general types of 
sources: incandescent filament lamps, standard arc 
lamps, and special gas-discharge lamps. Wherever pos- 
sible, a brief description of the type of associated elec- 
trical and optical equipment employed with each 
lamp has been included. 

The types of applications for which radiation 
sources have found actual or potential military use 
are quite numerous. In general, incandescent fila- 
ment lamps have been employed in systems of detec- 
tion and recognition, aircraft position indicators, sig- 
naling and other communication, reconnaissance, and 
nocturnal vehicular movement. Carbon-arc lamps have 
been used principally in reconnaissance work ; mer- 
cury-vapor lamps for ultra high-speed photography, 
target seeking, and several ultraviolet projects: and 
special gas-discharge souices for modulated commu- 
nication woi k. Because of security requirements, most 
of the projects falling under the categories just men- 
tioned have involved infrared sources, though ultra- 
violet radiation has been used in ceitain cases. A dis- 
cussion of all receivers and a detailed treatment of 
filters are beyond the scope of this chapter. 


52 INCANDESCENT FILAMENT LAMPS 

The incandescent tungsten filament lamp has been 
used to a greater extent as a radiation source in proj- 
ects covered by this report than any other type of 
lamp. There are several reasons for this: (1) the 
tungsten lamp is the most efficient producer of near 
infrared; (2) at higher filament temperatures, some 
ultraviolet is pioduced ; (3) all the radiation and elec- 
trical characteristics of tungsten are well-known ; 
(4) incandescent lamp design and production facil- 
ities are readily available. 

^Project Engineer, NDRC Contracit OEMsr-423, Nela Park 
Lamp Development Division, General Electric Company. 


^ General Characteristics 

The general nature of tungsten filament lamps is 
universally known. Basically, all such lamps are sim- 
ilar — variations in bulb size and shape, base, and fila- 
ment form are shown in the Design and Construction 
section following and keyed to particular lamps listed 
in Table 1. 

Many of the filament lamps listed in this report 
were originally developed for military service; certain 
others are standard lamps available before the war. 
All were developed and manufactured by the Lamp 
Department, General Electric Company [GE] (Con- 
tract OEMsr-423). The military applications are in- 
dicated in the right-hand column of the tables. 

Desigx and Construction 


The specific physical details of all tungsten filament 
lamps are expressed in terms of bulb size and shape, 
base, filament form and shielding, and lamp length. 



R RP ST 


Figure 1. Bulbs used in incandescent and conven- 
tional mercury lamps described in this chapter. 


Bulb shapes are indicated by one or more letters 
which designate the particular type of liulb involved. 
These types are shown in Figure 1. The numerical 
portion of the bulb designation indicates the nominal 


70 





INCANDESCENT FILAMENT LAMPS 


71 


Table 1. Filament lamps. 


Lamp 

No. 

Watts 

Volts 

Amp 

Rated 

life, 

hours 

Bulb 

Base 

Max 

overall 

length, 

inches 

Fila- 

ment 

form 

Shield 

Approx. 

max 

cp 

Approx, 
spread 
to 50% max 
H V 

Approx, 
color temp 
volts °K 

Application 

4560 

600 

24 

25 

25 

PAR-64 

Flex. 

lug. 

4 

C-13 

A 

600,000 

7° X 41° 

24 3250 
22 3150 
20 3050 
18 2950 

Automotive housing — 8-in. 
diam,5-7-mmCorningNo.2540 
laminated filter. IR spotlight 
for tank, DUKW and other 
vehicles. Figs. 11 and 12. 
Bibliography Nos. 1, 4, and 5. 

4560 

600 

28 

21.4 

25 

PAR-64 

Screw 

term. 

4 

C-13 

A 

600,000 

7° X 41° 

28 3350 

New lamp design. Supersedes 
24-volt No. 4560. Also used 
with IR filter XRX7D as 
metascope source for BuShips. 
Figs. 11 and 12. 

4561 

250 

13 

19.2 

25 

PAR-64 

Screw 

term. 

4 

C-2V 

A 

500,000 

41° X 2f° 

13 3350 

Automotive housing — 8-in. 
diam, 5-7-mm Corning No. 
2540 laminated filter. Landing 
boat IR spotlight Norfolk 
tests. Bibliography No. 2. 

4562 

250 

28 

8.9 

25 

PAR-64 

Screw 

term. 

4 

C-13 

A 

450,000 

5f° X 3° 

28 3300 

Automotive housing — 8-in. 
diam, 5-7-mm Corning No. 
2540 laminated filter. IR driv- 
ing tests. Bibliography Nos. 
1, 4, and 5. 

4541 

450 

24 

18.75 

25 

PAR-56 

Flex, 

lug. 

41 

• 

C-13 

D 

425,000 

7° X 4° 

24 3400 

Automotive housing — 6J-in. 
diam, 5-7-mm Corning No. 
2540 laminated filter. Jeep, 
amphibious jeep, misc. IR 
headlighting, and signaling. 
Figs. 11 and 12. Bibliography 
Nos. 1, 4, and 5. 

4541 

450 

28 

16.1 

25 

PAR-56 

Screw 

term. 

41 

C-13 

D 

450,000 

7° X 4° 

28 3300 
26 3250 
24 3150 
22 3050 

New lamp design. Supersedes 
24-volt, No. 4541. Also used 
with XRX7D filter as meta- 
scope source for BuShips. Figs. 
11 and 12. 

4540 

450 

13 

34.6 

25 

PAR-56 

Screw 

term. 

41 

C-2V 

D 

500,000 

61° X 51° 

13 3350 
12 3250 
11 3150 

10 30.50 

Automotive housing — 6|-in. 
diam, 5-7-mm Corning No. 
2540 laminated filter. For 2J- 
ton truck headlighting misc. 
spot and marker applications. 
Bibliography Nos. 4 and 5. 
Also used in XRX7D filter as 
metascope source for BuShips 
(spreader reduced beam cp to 
50%). 

4543 

100 

12.5 

8 

50 

PAR-56 

Screw 

term. 

41 

C-6 

None 

300,000 

2° X 31° 

12.5 3100 

Airborne Beacon-Air Corps- 
Kopp IR170 glass. Figs. 13 
and 14. Bibliography No. 13. 

4522 

250 

13 

19.2 

25 

PAR-46 

Screw 

term. 

3| 

C-2V 

A 

250,000 

51° X 4° 

13 3350 

Marker application for Bu- 
Ships. Bibliography No. 2. 

4523 

250 

28 

8.9 

25 

PAR-46 

Screw 

term. 

3| 

C-13 

A 

225,000 

7° X 5° 

24 3100 
28 3300 

Used with XRX7D filter as 
metascope source for BuShips 
‘and with No. 2540 for IR 
driving. Fig. 11. Bibliography 
Nos. 4 and 5. 


72 


SURVEY OF INFRARED SOURCES 


Table 1. Filament lamps {continued). 


Lamp 

No. 

V^'atts 

Volts 

.4 nip 

Hated 

life, 

hours 

Bulb 

Base 

Max 

overall 

length, 

inches 

Fila- 

ment 

form 

Shield 

Approx. 

max 

cp 

Approx, 
spread 
to50%max 
H V 

Approx, 
color temp 
volts °K 

Application 

4030 

40-30 

40 

30 

6-8 

6 4 

6.4 

6 2 

4.7 

120 

200 

at 7 V 

PAR-56 

3 Cont. 
lug. 

5i 

C -6 

C -6 

None 

30.000 

20.000 

7° x 2° 

6.4 2900 

7.4 3050 

6.4 2950 

7.4 3100 

Automotive housing — 65 -in. 
diam, approx. 8 -mm Corning 
No. 2550 and 5874 filters. 
Head-lamp adapters for 25 - 
ton truck, amphibian(DUKW) 
and other vehicles. UV and 
IR night driving. Fig. 11 . Bib- 
liography Nos. 4, 5, 6 , and 9. 

2400 

* 

4.5-35 

45 

35 

6-8 

6.4 

6.4 

7.0 

7.5 

120 

200 


31 Cont. 
lug. 

51 

C -6 

C -6 

None 

30.000 

20.000 

7° X 2° 

6.4 2800 
6.4 2950 

5§-in. composite metal-glass 
head lamp. 6 -in. diam, approx. 
8 -mm Corning No. 2550 and 
5874 filters. Head lamp adap- 
tors for jeep and amphibious 
jeep. UV and IR night driv- 
ing. Bibliography Nos. 4, 5, 
6 , and 9. 

4011 

30 

6-8 

6.2 

4.8 

300 

PAR-46 

Screw 

term. 

4 

C -6 

C 

35,000 

5^ X 2i° 

6.2 2900 

Automotive housing — Polar- 
oid IR plastic filter. Boat 
marker at Norfolk tests. Bib- 
liography No. 2 . 

4015 

30 

6-8 

6.2 

4.8 

300 

PAR-46 

Screw' 

term. 

2 | 

C -6 

c 

8,000 

22 ° X 2 ° 

6.2 3000 

Automotive housing — 65 -in. 
diam, 5-mm Corning No. 2540 
filter. Landing boat identifica- 
tion. Norfolk tests. Also used 
for UV driving. Fig. 15. Bib- 
liography Nos. 2 , 6 , and 9. 

4013 

25 

6-8 

6.2 


300 

PAR-46 

Screw 

term. 

4 

C -6 

None 

1,000 

38° X 16° 

6.2 2950 
6.2 2950 

Exp. marker — glider landing. 
Bibliography No. 15. 

4524 


6.0 

4.75 

400 

PAR-46 

Screw 

term. 

4 

C -6 

None 

80,000 

3§° X 1|° 

6.0 3050 

RCA Laboratories — experi- 
mental snooperscope. Bibli- 
ography No. 15. 

Spec. 

30 

6.0 

5.0 

300 

PAR-36 

(clear 

cover) 

Screw 

term. 

2 | 

C -8 

None 

80,000 

5° X 3° 

6.0 3150 

RCA Laboratories — experi- 
mental snooperscope. Bibliog- 
raphy No. 15. 

Spec. 

30 

6.2 

4.75 

25 

PAR-36 

(clear 

cover) 

Screw 

term. 

2 | 

C -6 

None 

50,000 

7° X 3° 

6.2 3050 

RCA Laboratories — experi- 
mental snooperscope, Bibliog- 
raphy No. 15. 

4501 


26 

5.3 

50 

PAR-36 

Screw 

term. 

2 | 

4CC-8 

None 

50,000 

6 ^° X 6 i ° 

26 3000 

Experimental tow-plane iden- 
tification. Glider towing. Bib- 
liography No. 15. 

Spec. 

250 

28 

8.8 

25 

PAR-36 

(frosted 

cover! 

Screw 

term. 

2 f 

C-13 

None 

7,000 

26° X 27° 

28 3300 

Experimental tow-plane iden- 
tification. Glider towing. Bib- 
liography No. 15. 

Spec. 

5.6 

4.5 

1.25 

75 

PAR-36 

Screw 

term. 

2 | 

C -6 

(Vert.) 

None 

1,500 
at 5.1 V 
on 4 dry 
cells 

22 ° X 21 ° 

4.5 3000 

65 -in. diam, 8 -mm Corning 
No. 2550 filter. Portable bat- 
tery-operated IR beach mark- 
ers. Fig. 16. Norfolk tests. 
Portable IR runway markers. 
Glider landing. IR driving. 
Bibliography Nos. 2, 4, 5, 
and 15. 

Spec. 

150 

12.8 

11.7 

200 

PAR-56 

Screw 

term. 

4f 

C -6 

None 

130,000 

7§° X 3^° 

12.8 3100 

Vehicle head lamps — Polaroid 
filter. Jeeps and misc. trucks 
for Eng. Bd. IR driving. 
Bibliography No. 15. 


♦Lamp bulb is not separately replaceable but is soldered into reflector of composite unit. 


INCANDESCENT FILAMENT LAMPS 


73 


Table 1. P''ilament lamps {continued). 


Lamp 

No. 

Watts 

Volts 

Amp 

Rated 

life, 

hours 

Bulb 

Base 

Max 

overall 

length, 

inches 

Fila- 

ment 

form 

Light 

center 

length, 

inches 

Approx. 

max 

cp 

Approx, 
color temp 
volts °K 

Application 


2,500 

65 

1 

38.5 

50 

T-24 

Mog. 

bipost 

111 

C-13D 

4 


65 3300 

70 3400 

24- and 36-in. searchlights. IR reconnais- 
sance and locomotive driving. Fig. 17. 
Bibliography Nos. 3, 7, and 8. 


4,000 

no 

36.4 

50 

T-32 

Mog. 

bipost 

14 

C-13D 

4 


110 3300 

Navy types exp. searchlights. IR shoreline 
reconnaissance. Bibliography No. 11. 


1,800 

28 

64.3 

10 

T-20 

Mog. 

bipost 

9| 

C-8 

curved 

6i 


28 3400 

See Sections 5.3.1 and 9.3.1. 


1,000 

115 

8.7 

25 

T-20 

Mog. 

bipost 

9| 

C-13D 

4 

2,850 

Perpendic- 
ular to plane 
of filament 

115 3050 

24-in. GE searchlight gold-plated re- 
flector. 2 layers 8 mm each Corning No. 
2550 filter. IR reconnaissance and loco- 
motive driving. Bibliography No. 3. 


250 

115 

2.2 

200 

G-30 

Med. 

screw 

5i 

C-5 

2* re 

400 

115 2850 

360 markers for Naval Aircraft Factory 
— IR homing beacon for carrier landing 
— IR lacquer filter. Bibliography No. 15. 


240 

24 

10 

100 

A-19 

Med. 

pref. 

4i 

C-2V 

1| 



Portable IR beacon for airborne land- 
ings. 360° Fresnel lens. Fig 18. Bibli- 
ography No. 15. 


150 

110 

1.4 

25 

T-8 

D.C. 

Bay. 

3| 

2CC-8 

It 

300 

Approx. 

spherical 

100 3100 

Motor-driven revolving beacon glider 
landing — IR lacquer. Fig. 19. Bibliog- 
raphy No. 15. 


100 

115 

0.87 

1,000 

R-40 

Med. 

screw 

6| 

C-11 


1,275 

115 2800 
125 2900 

IR runway marker for aircraft carriers. 
Approx. 8-mm Corning No. 2550 filter. 
Fig. 20. 


85 

10.6 

8 

100 

A-17 

Med. 

pref. 

3| 

4C-8 

Pd'e 


10.6 2950 

Airborne marker — A.G. F. Eng. Bd. Used 
in 360° Fresnel — Polaroid. Bibliography 
Nos. 13 and 15. 


75 

115 

6.5 

1,000 

T-12 

Med. 

screw 

lU 

C-8 


63 

115 2500 

IR wands for deck officer aircraft carrier 
landing. IR lacquer filter. Figs. 21 and 
22. Bibliography No. 15. 


cp 

or 

watts 

115 


1,000 

T-8 

Med. 

screw 

lU 

C-8 


32 

115 2500 

Wands for deck officer aircraft carrier 
landing. IR lacquer filter. Figs. 27 and 
28. Bibliography No. 15. 

40 

1196 

50 

cp 

12-16 

3.46 

300 

RP-11 

D.C. 

Bay. 

2i 

C-2V 

u 

50 — Spher- 
ical 4,000- 
in. reflector 
75°x4° 

12.5 3000 

Portable battery-operated revolving bea- 
con. Glider landing IR lacquer filter. 
Bibliography No. 15. 

310 


28 

0.91 

300 

S-11 

D.C. 

Bay. 

2i 

C-2V 

u 

32 

28 2950 

Beacon runway marker for aircraft land- 
ing. 4 with lacquer filter per marker. Figs. 
23 and 24. Bibliography Nos. 10 and 12. 

1045 

30 

5.9 

5.1 

30 

RP-11 

S.C. 

Bay. 

pref. 

2i 

C-6 

i 

5,300 in 
gold 

parabola 

5.9 3300 

Snooper-sniperscope production model. 
Bibliography No. 15. 

63 

3 

cp 

7 

0.64 

500 

G-6 

S.C. 

Bay. 

IWe 

C-2R 

t 

63 - 360° 
Horizontal 
band in 
cylindrical 
Fresnel 
lens 


Portable battery-operated runway 

marker. 360° Fresnel lens. Glider land- 
ing. IR lacquer filter. Fig. 25. Bibli- 
ography No. 15. 

1524 

21/6 

cp 

28 


300/ 

1,200 

GG-10 

S.C. 

Bay. 

Index- 

ing. 

2*4 

CC-6/ 

CC6 

' 

21—6 

28 2800 

Cowl flood and wing-tip marker lacquer 
filter. Aircraft landing — BuAer. Bibli- 
ography Nos. 10 and 12. 




74 


SURVEY OF INFRARED SOURCES 


Table 1. Filament lamps {continued). 


Lamp 

No. 

CP 

or 

watts 

Volts 

Amp 

Rated 

life, 

hours 

Bulb 

Base 

Max 

overall 

length, 

inches 

Fila- 

ment 

form 

Light 

center 

length, 

inches 

Approx. 

max 

cp 

Approx, 
color temp 
volts ®K 

Application 

14 


2.5 

0.30 

15 

G-3| 

Min. 

screw 


C-2R 


5 spherical 
cp 


Used in flashlights for IR signaling and 
airborne operations. Fig. 26. 

51 

1 cp 

6-8 

7.5 

0.22 

1,000 

G-3i 

Min. 

Bay. 


C-2R 

h 

1 spherical 
cp 


Used in flashlights for ground assembly, 
etc., on airborne operations. Fig. 26. 
Bibliography Nos. 13 and 15. 

53 

1 cp 

12-16 

14.4 

0.10 

1,000 

G-3J 

Min. 

Bay. 


C-2V 


1 spherical 
cp 


Used in flashlights for ground assembly, 
etc., on airborne operations. Fig. 26. 
Bibliography Nos. 13 and 14. 

PR2 


2.4 

0.50 

15 

B-3§ 

S.C. 

min. 

flanged 

U 

C-2R 

i 

j spherical 
cp 


Used in flashlights for IR signaling. 

PR3 


3.6 

0.50 

15 

B-3^ 

S.C. 

min. 

flanged 

U 

C-2R 

\ 

Ij spherical 
cp 

3.57 2805 

Used in flashlights for IR signaling. 



3 CONTACT LUGS 



SCREW TERMINAL 



ADMCDIUM 



MEDIUM SCREW 



MOGUL BIPOST 



DOUBLE CONTACT 
BAYONET 
CANDELABRA 


BAYONET 

CANDELABRA 

WITH 

PREFOCUSING 

COLLAR 


SINGLE CONTACT S C BAY INDEXING 
BAYONET 
CANDELABRA 


Figure 2. Bases used on incandescent and conventional mercury lamps described in this chapter. 


diameter in eighths of an inch. In the case of PAR 
type lamps, the reflector is an integral part of tlie 
lamp. On occasion, filter materials (lacquers) have 


been applied directly to the bulbs of certain lamps, 
as indicated in the Application column of the lamp 
tables. 




INCANDESCENT FILAMENT LAMPS 


75 


Base designations are given in Figure 2. The light 
center length gives the distance from the center of the 
light source to the point indicated in the list below, 
depending on the base used : 


Type of base 
All screw bases 
Medium and mogul prefocus 
Mogul bipost 
Medium bipost 
Prong 

Bayonet candelabra 
Medium bayonet 
SC or DC prefocus 

Miniature flanged 


Point for measure 
Bottom-base contact 
Top of base fin 
Shoulder of post 
Bottom of bulb (base end) 
Nut, washer or shoulder of 
base prong 
Top of base pins 
Top of base pins 
Plane of locating bosses of pre- 
focusing collar 
Plane of locating bosses 


The filament designation consists of a prefix letter 
to indicate whether a wire is straight or coiled, and a 
number to indicate the arrangement of the filament 
on the supports. Prefix letters include: 8 (straight), 
wire is straight or slightly corrugated; C (coiled). 



Figure 3. Filament forms used in incandescent 
lamps described in this chapter. 


wire is wound into a helical coil; CC (coiled coil), 
wire is wound into a helical coil and this coiled wire 
is again wound into a helical coil. The shapes of fila- 
ments are shown in Figure 3. 

Operating Characteristics 

Power Supply. Tungsten filament lamps can be 
operated from either a-c or d-c, except for certain re- 
strictions where carrier-wave detection and current 
modulation are involved. The power supply for the 
sources listed in Table 1 depends on the military ap- 
plication, and is determined by the work cycle involved 
for the particular source, the voltage and current re- 
quired, and the type of available power for other 
electrical equipment. Sources for use on aircraft are 
designed to operate from the standard electrical power 
supply of the plane; sources used in airborne opera- 
tions are usually powered from batteries; and large 
sources for reconnaissance operations have been oper- 
ated from mobile engine-generator sets. 

Modulating Equipme7it. Filament lamps were not 
used for current-modulated communication. 



Figure 4. Characteristic curves for large gas-filled 
tungsten filament lamps. 


Electncal Characteristics. The electrical character- 
istics of filament lamps are determined by the fila- 
ment, which is designed to operate at a particular 
temperature and give acceptable life for service under 
definite conditions of voltage, vibration, shock, etc. 
Candlepower, current, wattage, color temperature, 
and life values for the rated voltage of each lamp are 
given in Table 1. Values of resistance, current, light 
output, wattage, and efficiency for operation at other 
than the rated voltage of a large gas-filled tungsten 
lamp are shown in Figure 4. The average effect of 
voltage on life is also indicated. It should be remem- 
bered, however, that this life curve cannot be expected 
to apply with great accuracy to a single lamp or a 











LIMNS PFR k 

TOPI 

TO 













^AJT 

- 


























































0 20 40 60 80 idd 120 141) 

PER CENT LIFE 


Figure 5. Depreciation throughout life for a 115- 
volt, 1.7 ampere, general service, tungsten filament lamp. 


small group of lamps, because normal probability 
functions apply just as they do to lamps operated at 
rated voltage. 

The electrical characteristics change some through- 
out the life of a tungsten filament lamp. Because of 
gradual evaporation, the filament becomes thinner 
and thus has a higher resistance, thereby using less 
current and wattage. The radiant energy output de- 


76 


SURVEY OF INFRARED SOURCES 



COLOR TEMPERATURE IN DEGREES K 


Figure 6. The visible output efficiency versus fila- 
ment color temperature. 

creases during life because of lowering filament tem- 
perature and bulb blackening, at a rate which depends 
on the temperature at which the filament is operated 
and the ratio of watts per unit internal volume of 
the lamp. Certain relatively low wattage PAR type 
lamps have almost the same light output at the end 


of rated life as when they are first placed in service. 
This is due to an early gain in efficiency because of 
filament seasoning. Figure 5 shows the depreciation 
curves for a 750-hour rated life, 1.7-ampere filament 
in a PS-30 bulb. 

Radiation Characteristics 

Spectral Distribution. The spectral distribution of 
incandescent tungsten filament lamps is continuous 
and, in general, has the shape of the familiar black- 
body radiation curve. Within the visible spectrum, 
the distribution corresponds closely to that from a 
black body at the color temperature of the lamp. 

The luminous efficiency of a filament lamp increases 
as the temperature is raised. Figure 6 shows the effi- 
ciency in lumens per watt for a group of different 
lamps ranging in color temperature from 2750 K to 
3475 K. This relationship can be expected to hold for 
most clear or frosted-bulb tungsten lamps. The candle- 
power in a given direction will increase at the same 
rate for a given type of lamp and filament construc- 
tion. Figure 7 gives the spectral radiant intensity per 
1,000 candles for five color temperatures. Approxi- 
mate values for other color temperatures can be ob- 
tained by interpolation. The color temperature is fairly 
close to the actual filament temperature. The approxi- 
mate color temperature for the various lamps used 
in NDRC projects is given in Table 1. 

Brightness. The filament brightness of an incandes- 
cent lamp depends on its temperature, the latter being 



Figure 7. Spectral distribution curves for tungsten filament lamps at five color temperatures. 


INCANDESCENT FILAMENT LAMPS 


77 



Figure 8. Brightness versus temperature for well- 
aged tungsten. 


selected by the lamp designer to give the desired life 
for the service intended. Figure 8 gives the brightness 
of well-aged tungsten at various temperatures, and 
can be used to indicate the approximate brightness 
of coiled filaments, particularly in the range 2700 to 
3000 K. Of course, the brightness of the filament 
will be less near a support wire or lead, because such 


members drain heat from the filament by conduction. 
In the case of inside frosted, enameled, or colored 
glass bulbs, the brightness is much less than that of 
the filament. 

Spatial Distribution. The energy distribution in 
space about filament lamps depends on the bulb shape 
and finish, and the filament construction. The beam 
dimensions of PAR type lamps are indicated in the 
tabular data; such lamps have different horizontal and 
vertical distributions because of the flutes on the lens 
or cover plate and the use of a bar filament. Projec- 
tion, airway beacon, and similar lamps used in NDRC 
projects have maximum output perpendicular to the 
plane of the filament. 

Radiation Charactekistics in 
Associated Optics 

Spectral Energy Distribution. Applications of fila- 
ment lamps for the utilization of white light result in 
spectral distributions fairly similar to those for the 
light source, because the reflector and lens systems 
used in such applications are reasonably nonselective 
in reflectance and transmission. The distribution of 
the output of ultraviolet [UV] and infrared [IR] 



Figure 9. British source and telescope equipment for infrared night driving. 


78 


SURVEY OF INFRARED SOURCES 




Figure 11. 34-ton 4x4 amphibian with 450- watt infra- 
red head lamps, 600- watt spot lamp (filterremoved) and 
low-wattage running lights. 


Kellner- Schmidt system (see Table 1 and Section 
9.3.1 for other details). The mirror has a radius of 
8 inches, clear aperture 8% inches, focal length 4.42 
inches, aperture ratio //0.53. The output is 800,000 
beam candlepower in a solid angle 1.5 by 25 degrees. 
Astigmatism is introduced into the corrector plate to 
smear the image of the coiled filament. 

A British infrared beacon has also been used as a 
metascope source at the Engineer Board, Fort Belvoir. 
The beacon employs a 200-watt, 12-volt lamp and is 
operated from lightweight storage batteries. The IR 

f 1 


Figure 12. Infrared equipment on a medium tank, 
consisting of four 450-watt IR projectors on the front 
and one 600- watt IR searchlight on left top of turret. 
Binocular image tubes are mounted in a socket welded 
to the driver’s direct-vision slot visor. Source of power 
is Waukesha multi-fuel model 234 T.G.U. engine gener- 
ator mounted in left rear sponson. 

filter is incorporated in polyvinyl alcohol. British IR 
driving equipment (Figures 9 and 10), using 36-watt, 
12-volt lamps in headlight reflectors, was tested by the 
General Electric Company. These sources also have 
polyvinyl alcohol filters.® 

Sources used in the triple-mirror development for 
Army aircraft landing include a special 3-ampere 
tungsten lamp with 225 candlepower, and the small 
grain-of-wheat lamp giving 0.29-0.35 candlepower at 
3 volts (color temperature 2510 K). 

Photographs of many incandescent lamp applica- 
tions are given in Figures 11 through 26. 

^ 2 ^ Recommended Filament Lamps for 
Various Military Projects 

The lamps listed in Table 1 are not necessarily the 
best sources for the projects indicated. In some cases, 
several lamps were tried (and are listed) before the 


Figure 10. The equipment of Figure 9 mounted on 34- 
ton truck. There are four 36- watt projectors; binocular 
image tubes mounted on springs in frame suspended 
from windshield frame and front bow. Binoculars are 
fastened to driver’s head with web harness. The power 
source is a 12-volt battery strapped on the left sponson. 


units can be calculated from the spectral transmission 
curve of the filters employed. Beam dimensions of 
filtered outputs are comparable to those given for the 
light source in the case of PAR lamps, because filters 
with little diffusion have been employed in most ap- 
plications of UV and IR projected beams. 


Miscellaneous Filament Sources 


A Kellner-Schmidt projector (design 3A) was de- 
veloped by the Institute of Optics, University of 
Rochester (Contract OEMsr-1219), as a visible or 
infrared source for aerial photography work by the 
AAF, Wright Field. The 1,800-Avatt lamp has a single 
filament curved to fit the optical requirements of the 




INCANDESCENT FILAMENT LAMPS 


79 



Figure 13. Components of airborne beacon, includ- 
ing collapsible mast, stakes, guy wires, and twin sources. 


best lamp or combination was selected. Also, improve- 
ment in infrared filter and receivers (such as the re- 
cent high-voltage RCA model) should make certain 
previous recommendations obsolete. 

1. Infrared Night Driving 

Head lamps: Two 150-watt, PAR-56 special 
lamps, with beam pattern similar to automotive 
practice. Filtered with three layers of XR7X25. 
Spotlight : One Xo. 4522 or 4523 250-watt, PAR- 
46, 13-volt or 28-volt lamp filtered with three 
layers of XR7X25. 

2. Ultraviolet Night Driving with Auto collimating 
Fluorescent Reflector ButtonsF^ 

Head lamp: One Xo. 4015, with 586 and 587 
glass filter mounted on top of windshield just 
above drivePs line of vision. 

3. Night Shore Reconnaissance. 

Source: Two 4,000-watt, 115-volt T-32 lamps, 
each in 24-inch reflector with OSU or Polaroid 
filter. (Both the twin 24-inch carbon-arc search- 
lights and the 60-inch antiaircraft arc search- 
light were used for shore reconnaissance, but the 
contract was terminated before the relative mer- 
its of arc-lamp and filament-lamp searchlights 
were completely established.) 

4. Nocturnal Gun Ranging.^ 

Source : Same as for Xight Shore Reconnaissance. 

5. Night Towing and Landing of Gliders.^^^ 
Markers for Tow-Plane : Three special PAR-36, 
250-watt, 13-volt or 28-volt wide conical distrib- 
ution lamps and infrared filter. Used to locate 
tow plane for glider pilot. 


I 


1 



Figure 14. A daytime view of the airborne beacon 
assembled from the parts of Figure 1. 



Figure 15. The No. 4015 lamp and filter (in various 
sizes) used for night driving with ultraviolet radiation. 

Landing Location Markers : B-9 airborne beacons 
using lamp Xo. 4543 and infrared filter. 
Markers for Landing Strip : Had not been estab- 
lished. 

6. Night Railway Operation.^’’^’^ 

Head lamp : One 2,500-watt, 65-volt lamp, in 24- 



80 


SURVEY OF INFRARED SOURCES 



Figure 16. Improvised IR beach marker for recon- 
naissance test using PAR-36 4.5-volt, 1.5-ampere special 
lamp and Corning No. 2540 filter. 



Figure 17. The 36-inch IR searchlights and 12-inch 
Schmidt receiver. 


inch reflector and infrared Alter for locomotive 
headlamp. 

7. Night Landing of Carrier AircraftN’'^^ 

Runway ]\rarkers: Pattern of 4 No. 310 filtered 
lamps, one at each corner of a 2-foot square. 



Figure 18. Experimental airborne beacon on 8-foot 
mast built for ETO. 


Markers spaced on 100-foot centers down the 
sides of a 100x600-foot runway. 

LSO “Wands 15-cp coated lamps spaced along 
each of two light metal irands for hand signaling. 
One similar strip suspended vertically from 
Landing Signal Officer’s neck for orientation. 
Navigation Lamps on Plane: Existing lights, fil- 
tered. 

Cowl Illumination Lamp: One No. 1524 GG-10 
lam]) and filtered glohe to illuminate cowl of 
plane for ])ilot orientation. 





INCANDESCENT FILAMENT LAMPS 


81 



Figure 19. Experimental rotating IR beacon using 
115-volt, 150-watt lamp for glider landing project. 



Figure 20. Infrared marker using 115-volt, 150-watt 
R-40 lamp for carrier landing project. 



Figure 21. The first experimental IR “wand” for 
use by the Landing Signal Officer. 



Figure 22. The second type of IR “wand” for the 
Landing Signal Officer. 



Figure 23. A wire-frame multiple-source beacon to 
mark runways for IR night landing of aircraft. 


Approach or Altitude Lamp : Two 21-cp bar fila- 
ment lamps in fixture developed by the Johnson 
Foundation, University of Pennsylvania, under 
NDRC Contract OEMsr-1075. 

8. Snooperscope-Sniperscope}^^ 

Source: No. 1045 lamp in 5-inch gold-plated 
reflector and Polaroid filter. 

9. Airborne Beacon}^ 

Two No. 4543 100-watt, 12.5-volt PAR-56 spot 
lamps with infrared filter. 

10. Hidden Japanese Defenses}"^ 

At the single trial of this project, one 1,500-watt, 
32-volt, T-24 filament lamp was used in a 24- 
inch searchlight with OSU filters. 



Figure 24. Same as Figure 23 except extra sockets 
on wood panel permit flexibility of source pattern. 


82 


SURVEY OF INFRARED SOURCES 



Figure 25. Experimental battery-operated infrared runway marker for glider landing. 



Figure 26. Left and center, 360-degree hand infrared 
source; right, filter-coated lamp with output adjustable 
by resistor head of battery case. 


53 CONVENTIONAL ARC LAMPS 

^ Carbon-Arc Lamps 

Genekal Characteristics 

The conventional carbon arc has been used by Sec- 
tion 16.5 both as a visible source and as a concentrated 
source for long-range projection of IR. The general 
designations, manufacturers, and military applica- 
tions are given in Table 2. 

The two visible light sources were used by the In- 
stitute of Optics, University of Rochester. The Sim- 
plex arc lamp was employed in an 11-inch reflector; 
the Westinghouse carbon-arc assembly, in an 18-inch 
parabolic reflector mounted in a nacelle for aircraft 
installation and with remote control. 

The 24-inch arc IR unit was adapted from the 
standard Navy searchlight of the same size. Two such 
searchlights were mounted in tandem on a single base, 
with mechanism for directing the pair simultaneously 
for azimuth and elevation (Figure 27). The operators 


CONVENTIONAL ARC LAMPS 


83 


seat and the manual aiming controls are located in 
the center behind the two searchlight drums. A re- 
sistance box was located at the rear of each searchlight 

Table 2. Carbon-arc lamps. 


Source Development Where Military 

designation auspices developed Mfd. by application 


Simplex 
arc lamp 

BuAer 

Simplex 

Co. 

Simplex 

Visible light 
for sea 
search 

Westinghouse 

carbon-arc 

assembly 

BuAer 

WE 

WE 

Visible light 
for sea 
search 

24-in. carbon- 
arc search- 
light 

BuShips 

GE 

GE 

IR recon- 
naissance 

60-in. carbon- 
arc search- 
light 

BuShips 

GE 

GE 

IR recon- 
naissance 


and turned in azimuth with the units. Mounting was 
also provided for an infrared telescope conveniently 
located for the seated operator. Eleven-millimeter 
high-intensity carbons (made by National Carbon 
Company) were employed.^^ 


The 60-inch arc IR unit consisted of a standard 
antiaircraft searchlight of this size modified by ap- 
plication of Polaroid IR filter material to the inside 
of the cover glass. The unit was used with the stand- 
ard remote control station, the latter changed to ac- 
cept an infrared telescope. The control station was 
interconnected electrically with the searchlight to cor- 
relate its aim with that of the telescope. High-intensity 
carbons made by National Carbon Company were em- 
ployed ; the positive carbon was 16 millimeters in diam- 
eter, the negative 11 millimeters. The normal arrange- 
ment of power plant, searchlight, and control station 
is shown in Figure 28.^^ 

Operating Characteristics 

Power Supply. The Simplex arc lamp was powered 
from a d-c generator. Both the 24-inch and the 60- 
inch carbon-arc IR units were run (but not simulta- 
neously) from the mobile d-c engine-generator set 
which is standard equipment for the 60-inch antiair- 
craft searchlight (see Figure 28, left). 

Modulating Equipme7it. None was employed with 
these conventional arc lamps. 



Figure 27. Twin 24-inch IR searchlights used with filament and arc lamps for shoreline reconnaissance. 


MM 



84 


SURVEY OF INFRARED SOURCES 



Figure 28. (Left to right) The power plant, 60-inch IR arc searchlight, and control station employed for nocturnal 
shoreline reconnaissance. 


Electrical Characteristics. The well-known charac- 
teristics of arc lamps (change of current with voltage, 
etc.) apply to these sources (Table 3). Characteristics 
of this nature, for the particular IR arc searchlights 
employed, have not been obtained because they have 
always been operated at rated voltage and current. 


Table 3. Arc-lamp characteristics. 


Source 

Volts 

Amperes 

Simplex arc lamp 



Westinghouse carbon-arc assembly 

27 

45 

24-in. arc searchlight (each unit) 

65 to 70 

75 to 80 

60-in. arc searchlight 

80 

150 


Radiation Charactekistics 

Spectral Distribution. The approximate spectral dis- 
tribution curves for the high-intensity arcs employed 
in the 24-inch and 60-inch IR searchlights are given 
in Figures 29 and 30. The output is a combination 
of line, band, and continuous spectra. Since the car- 
bons are replaced as consumed and the reflectors are 
periodically cleaned, there is no change in spectral 
quality during the normal life of the searchlight (see 
Table 4 for other characteristics). 

Mercury -Vapor Lamps 

General Characteristics 

A mercury-vapor lamp is one in which light and 
other radiation is produced by the excitation and ion- 


ization of mercury atoms. Thus the lamp consists of 
an envelope to contain the mercury vapor and two 
or more electrodes to deliver power for starting and 
maintaining the arc discharge. Mercury-vapor lamps 
are versatile as a type of light source. As will be 
pointed out later, variations in the design of the 
lamp (vapor pressure, current, voltage, and the like) 
can be used to help control the distribution of energy. 

Standard mercury lamps are designated by a letter- 
number combination. Lamps having the same numer- 



Figure 29. The approximate spectral distribution of 
the carbon-arc lamp in the 24-inch searchlight. 


ical designation have identical voltage, current, and 
power requirements, and therefore each can be em- 
ployed with a given design of transformer. For ex- 
ample, C-H4 and E-II4 lamps can each be operated 
from the same transformer, because all mercury lamps 




CONVENTIONAL ARC LAMPS 


85 


Table 4. Radiation characteristics in associated optics. 


Source 

Candlepower 

millions 

Beam spread 
to 50 per cent 
maximum 

Filter 

Half-beam 

candlepower 

Visual 

range 

yards 

Simplex arc 

7 - 10.5 

3°x3° 


Used as 
visible source 


Westinghouse carbon-arc assembly 

30 

3°x3° 


Used as 
visible source 


24-in. twin IR searchlight (both units) 

130 

l°xl° 

4 layers 
XR7X25 

1,550,000 

475 

60-in. IR searchlight 

700 

30 3 0 

4 -^4 

7 layers 
XR7X25 

1,200,000 

125 


with a in their designation have identical mer- 
cnry-arc elements. The initial letter simply indicates 
modifications for different bulb shapes, burning posi- 
tions, or type of outer glass employed. 



Figure 30. The approximate spectral distribution of 
the carbon-arc lamp in the 60-inch searchlight. 


Each mercury lamp must have auxiliary equipment, 
which often consists simply of the proper size and 
type of transformer to provide the required electrical 
values for lamp starting and operation. Separate 
windings for two lamps are sometimes located on the 
same core, but provision must be made to ballast each 
lamp because of the negative volt-ampere character- 
istics of all discharge sources. 

Illustrations of the bulbs and bases listed in Tables 
5 and 6 are among those given earlier in this report, 
except that details of the positioning of the mercury 
element in the J-H4 are shown in Figure 31. The 4- 
watt germicidal lamp has a bent-tube construction 
which makes the lamp approximately 1 inch in width ; 
it has a radio-type 4-prong base. 

Opeeating Chaeactekistics 

Power Supply. Because all mercury lamps have a 
negative resistance characteristic, each lamp must be 
provided with suitable ballast equipment to prevent 
the arc current from reaching destructive values. In 


Table 5. Development and manufacturing data on mercury-vapor lamps. 


Source 

Auspices for 

NDRC project 

Development* 

institution 

Original purpose 

Military 

applications 

Mfd. 

by 

C-H4 

Army and Navy 

GE 

Near UV spotlight 

Night driving 

Night landing 

GE 

E-H4 

Army and Navy 

GE 

Near UV floodlight 

Night driving 

Night landing 

GE 

J-H4 

Army and Navy 

GE 

Night landing 

Night driving 

Night landing 

GE 

A-H6 

BuAer 

GE 

Photochemical and general lighting 

Sea search 

GE 

A-HIO 

Army and Navy 

GE 

Night landing 

Night landing 

GE 

B-HIO 

Army and Navy 

GE 

Night landing 

Night landing 

GE 

4-W BuShips 

Germicidal 

GE 

Germicidal source 

Metascope source 

GE 


*A11 but night driving are NDRC development projects by Institute of Optics, University of Rochester; night-driving project by RCA (instruments) 
and from GE. 


86 


SURVEY OF INFRARED SOURCES 


Table 6. Design and construction of mercury-vapor lamps. 


Source 

Bulb 

Base 

M.O.L. 

(in.) 

Number of 
electrodes 

C-H4 

PAR-38 

Admedium screw 


3 

E-H4 

PAR-38 

Admedium screw 

51 

3 

J-H4 

PAR-56 

Screw terminal 


3 

A-H6 

T-2 

^in. sleeve 

31 

2 

A-HIO 

PAR-56 

Screw terminal 

3f 

2 

B-HlO 

PAR-56 

Screw terminal 

51 

2 

4W 

Germicidal 

T-4 

Radio 4-terminal 

5| 

2 


most cases, this type of control can be achieved by 
employing a transformer specifically designed for high 
reactance, with sufficient open-circuit voltage to ini- 
tiate the discharge and a reduced value when the arc 
current builds up to the proper value. Where stand- 
ard voltage a-c power supplies (115 or 230 volts, 60- 
cycle) are available, standard transformers can be 
employed. Special designs must be used for other volt- 
age and frequency installations. High-pressure mer- 
cury sources (all the lamps discussed in this section 
except the 4-watt germicidal lamp) do not give best 



Figure 31. Construction details of the 100-watt 
J-H4 mercury lamp. 


performance on d-c power supplies because of the re- 
duction in overall efficiency due to higher wattage 
losses in the resistance ballast circuit. Too, the diffi- 
culty of providing the open-circuit voltage necessary 
for starting often rules out the use of ordinary mer- 
cury-vapor lamps on direct current. 

The starting of C-H4, E-H4, and J-H4 lamps is 
accomplished by a third or starting electrode close to 
one of the main electrodes and connected to the other 
main electrode by a small resistor. The A-H6 is a 2- 



Figure 32. The Z-12146 magnetic starter used for 
the A-HIO and B-HIO mercury lamps. 


electrode lamp and is started by applying 1,200 volts 
(the open-circuit value) across the electrodes. The 
discharge in A-HlO and B-IIlO lamps is initiated by 
the use of a magnetic starter shown in Figure 32. 
The starter (GE Catalog No. Z-1214G) is connected 
as in Figure 33. 

The voltage surge across the lamp when the starter 
contacts open starts the discharge, and the lamp cur- 
rent is sufficient thereafter to hold the contacts open 
magnetically. The 4-watt germicidal lamp can be 



H-l 

TRANS 

Z-12146 


LINE 

' ■ 







Figure 33. Diagram of magnetic starter connection. 

started with a push button arrangement or standard 
FS-5 fiuorescent starter to provide the cathode pre- 
heat time required. The standard 115-volt, 60-cycle 
ballast for this lamp carries the GE Catalog No. 58G- 
825; the ballast is designed to supply the proper cath- 
ode current during the short preheat interval. 

Because the 1,000-watt, A-H6 lamp has such a small 
volume, it is necessary that water be passed over the 
lamp with sufficient velocity to prevent the formation 
of steam bubbles on the surface of the quartz tube. 


iji iiiiiiniini 



CONVENTIONAL ARC LAMPS 


87 


Table 7. Electrical characteristics of mercury-vapor lamps. 


Lamp 

Lamp 

watts 

Overall 
watts 
(std trans) 

Rated 

life* 

(hours) 

Open circuit 
volts from 
trans (sec) 

Lamp 

(volts) 

Lamp 

starting 

(amp) 

Lamp 

operating 

(amp) 

Starting time 
to 

full output 

C-H4 

E-H4 

J-H4 

100 

123 

1,000 

245 

130 

1.3 

0.9 

3-8 min 

A-H6 

1,000 

1,095 

75 

1,200 

840 

2.5 

1.4 

4 sec 

A-HIO 

B-HIO 

400 

450 

20 

220 

137 

5.0 

3.2 


4-W 

Germicidal 

4 

5 

1,000 

Line 

values 

58 


0.8 



*Under specified test conditions as to frequency of starting, voltage, auxiliary equipment, and so forth. 


Table 8. Spectral 

distribution of radiation from 

100- watt C-H4 and 

E-H4 mercury 

projector lamps.* 

Wavelength 

Microwatts 

per sq cm at 

(center of band) 

10 ft from front of lamp 

(Angstroms) 

C-H4 

E-H4 

2,972 

0.007 


3,022 

0.097 


3,075 

0.081 

0.007 

3,131 

4.23 

0.931 

3,192 

0.85 

0.222 

3,255 

1.31 

0.375 

3,322 

7.41 

2.05 

3,394 

2.89 

0.863 

3,472 

2.85 

0.894 

3,556 

3.43 

1.03 

3,648 

150. 

42.4 

3,745 

8.01 

2.28 

3,852 

5.39 

1.53 

3,931 

5.49 

1.575 

4,049 

69.0 

19.4 

4,175 

8.04 

2.31 

4,358 

124. 

34.9 

4,560 

6.56 

1.85 

4,742 

6.31 

1.79 

4,947 

7.84 

2.19 

5,188 

7.52 

2.07 

5,461 

156. 

43.5 

5,780 

149. 

40.4 

6,143 

13.0 

3.48 

6,587 

13.5 

3.63 

7,105 

18.4 

4.85 


C-H4 running at 131.7 lamp-volts and 0.855 lamp-ampere. 


E-H4 running at 135.3 lamp-volts and 0.843 lamp-ampere. 

Data by B. T. Barnes, Lamp Development Laboratory, GE, Nela Park, 
Cleveland. From pages 62, 63, Book 58. 

*NOTE: These two lamps use the same type of mercury element and the 
spectral characteristics of each should be quite similar. Since intensities 
were measured on the axis, the lack of data for the E-H4 at 2972 and 3022 
Angstroms is due to cell sensitivity rather than absence of radiation. 

The spectral characteristics of the J-H4 should also be very similar to 
the C-H4 and E-H4. Candlepower of C-Ii4 17,5.50, of E-H4 4.8.30. 


This is accomplished by using a glass or quartz water 
jacket around the lamp, with a very small radial 
clearance to restrict the water flow. In approved de- 
signs, enough velocity results with a flow of about 3 
quarts per minute to prevent steam formation. 

Modulating Equipment. The mercury-vapor lamps 
described in this section of the report have not been 
used by Section 16.5 with modulating equipment for 
communication purposes. 

Lamp Lifes. The life of most mercury lamps is in- 
creased as the hours of burning per start become 
greater. In the case of C-II4 and E-H4 lamps, the 
rated life given in Table 7 is based on specifled test 
conditions with the lamps turned on and restarted no 
oftener than once every 5 burning hours. The A-H6 
life rating is based on tests employing 25-minute 
burning periods. With the lower ratio of total burning 
hours per start found in many military application.s, 
the life will be less. In the case of the A-II6, the life 
may not be more than 25 hours on very short burning 
periods, such as 3 to 5 minutes. 

Radiation Chaeacteristics 

Spectral Disinhution. Light and radiant energy 
result from mercury-vapor lamps because of energy 
transitions (electron displacements) in the ionized 
mercury atoms. Radiation of particular wavelengths 
is produced which corresponds to the resonant fre- 
quency of the atom, which in turn is dependent on 
the degree of electron displacement. At low pressures, 
such as found in the 4-watt germicidal lamp, the 
radiation output appears almost entirely as energy 
concentrated at these various resonant wavelengths, 
giving a line spectrum. At high pressures, such as the 
no atmospheres in the A-II6 when lighted, the radi- 


I 


88 


SURVEY OF INFRARED SOURCES 



ANGLE OFF AXIS IN DEGREES 

Figure 34. Candlepower distribution of a GE Mer- 
cury Mazda C-H4 projector spotlight, lamp operating 
with 115-volts, 60 c a-c on input side of auxiliary ; lamp 
rotating. Bulb is PAR-38 inside aluminized, stippled 
lens; No. 2 photometer at 25 feet. 



0 10 20 30 40 50 60 70 


ANGLE OFF AXIS IN DEGREES 

Figure 35. Candlepower distribution of a GE 
Mercury Mazda E-H4 projector floodlight, lamp oper- 
ating with 115-volts, 60 c a-c on input side of auxiliary; 
lamp rotating. Bulb is PAR-38 inside aluminized, cross- 
hatched lens; No. 2 photometer at 15 feet. 


Table 9. Spectral distribution of radiation from type H-6 water-cooled capillary lamn. 


Microwatts Microwatts Microwatts 


Wavelength 

band 

(Angstroms) 

per cm2 ^t 

1 meter 

quartz glass 

jacket jacket 

Wavelength 

band 

(Angstroms) 

per cm2 at 

1 meter 

quartz glass 

jacket jacket 

Wavelength 

band 

(Angstroms) 

per cm2 at 

1 meter 

quartz glass 

jacket jacket 

2216-2233 

0.003 


3291-3360 

94. 

44.5 

7259-7610 

62.0 

61.3 

2233-2250 

0.020 


3360-3434 

65.6 

36.5 

7610-8000 

63.1 

61.5 

2250-2268 

0.077 


3434-3514 

52.6 

35.2 

8000-8450 

65.4 

64.0 

2271-2289 

0.385 


3514-3601 

48.1 

36.8 

8450-8900 

70.4 

67.8 

2293-2312 

1.32 


3601-3696 

300. 

251. 

89C0 -9390 

71.3 

67.8 

2313-2333 

2.06 


3696-3798 

175. 

150. 

9390-9880 

74.4 

61.6 

2328-2348 

1.24 


3798-3902 

95. 

87. 

9880-10410 

130. 

111. 

2343-2364 

3.10 


3902-3960 


40.8 

10410-10960 

77.6 

69.8 

2367-2389 

10.8 


3960-4019 


46.3 

10960-11560 

95.3 

74.2 

2388-2410 

12.8 


4019-4079 


193. 

11560-12170 

65.2 

35.3 

2411-2434 

14.3 


4079-4142 


99. . 

12170-12800 

58.6 

32.3 

2435-2459 

15.3 


4142-4209 


65. 

12800-13440 

57.1 

24.2 

2452-2477 

21.2 


4209-4280 


78. 

13440-14060 

60.0 

8.7 

2470-2496 

27.4 


4280-4354 


159. 

14060-14550 

13.3 


2498-2524 

23.6 


4354-4431 


281. 

14550-15130 

13.7 


2524-2550 

9.5 


4431-4516 


93. 

15130-15710 

25.0 


2550-2578 

0.033 


4516-4605 


62.6 

15710-16290 

24.8 


2578-2607 

0.80 


4605-4696 


51.6 

16290-16850 

28.0 


2607-2638 

7.36 


4696-4789 


56.6 

16850-17390 

38.1 


2638-2671 

19.1 


4789-4892 


60.4 

17390-17920 

23.4 


2671-2705 

28.9 


4892-5002 


45.5 

17920-18430 

18.9 


2705-2741 

34.6 


5002-5123 


35.6 

18430-18930 

8.9 


2741-2780 

41.4 


5123-5252 


35.9 

18930-19420 

0.3 


2780-2820 

48.8 


5252-5388 


55.0 




2820-2861 

48.0 


5388-5536 


396. 

20350-20810 

2.7 


2861-2904 

57.6 


5536-5691 


134. 




2904-2949 

48.3 

. 

5691-5863 


273. 

21650-22080 

6.1 


2949-2998 

106. 

3.1 

5863-6043 


92. 




2998-3050 

116. 

6.8 

6043-6245 


54.4 

22910-23320 

3.9 


3050-3106 

76. 

7.7 

6245-6470 


50.5 




3106-3165 

173. 

31.5 

6470-6705 


51.5 

24190-24570 

0.85 


3165-3226 

117. 

29.1 

6705-6960 


55.9 




3226-3291 

76. 

26.5 

(>960-7250 


59.4 






^ I 


CONVENTIONAL ARC LAMPS 


89 


atioii tends to become continuous, or the line spectrum 
is superimposed on a continuous spectrum. 

Spectral distribution data are given in Tables 8 
through 11 for the mercury-vapor lamps described in 
this report. In the case of projector-type lamps (PAR- 
38 and PAR-56) used for visible applications, the 


values given represent the output of a complete unit. 
For ultraviolet and infrared energy applications, the 
lamp output must be modified by the spectral trans- 
mission characteristics of the filter. 

Brightness. The maximum brightness of conven- 
tional mercury lamps is in the direction perpendicular 


T.\ble 10. Spectral intensities perpendicular to axis of 400-watt, ribbon-seal, air-blast cooled mercury-vapor lamp. 


Primary volts = 115 
Primary watts = 450 
Lamp amp = 3.6 
Lamp volts =120 
Lamp watts = 400 
F-vitons per steradian = 283,000 


Candlepower = 1,660 

Lumens = 17,100 

Brightness = 64 candles per mm^ 

Color coord.: A = .324 Y = .365 

Color temp = 5870 K -f 38 m.p.c.d. 


Wavelength 

band 

Principal 

lines 

Milliwatts 
per steradian 

Wavelength 

band 

Principal 

lines 

Milliwatts 
joer steradian 

2198-2214 


49 

4431-4516 


48 

2214-2230 


67 

4516-4605 


34 

2230-2246 


85 

4605-4696 


32 

2246-2262 

2259-61 

101 

4696-4789 


35 

2262-2279 


110 

4789-4892 


45 

2279-2298 


116 

4892-5002 

4916 

56 

2298-2317 

2302-04 

130 

5002-5123 


33 

2317-2337 

2323 

110 

5123-5252 


34 

2337-2358 

2353-54 

129 

5252-5388 


60 

2358-2379 


128 

5388-5536 

5461 

1,070 

2379-2401 

2399-401 

149 

5536-5691 


no 

2401-2424 


98 

5691-5863 

5770-91 

1,410 

2424-2448 

2447 

65 

5863-6043 


94 

2448-2472 

2464 

107 

6043-6245 

6234 

64 

2472-2498 

2482-84 

290 

6245-6470 


65 

2498-2524 


126 

6470-6705 


70 

2524-2550 

2535-39 

340 

6705-6960 

6716 

96 

2550-2578 

2576 

450 

6960-7250 

7082-92 

95 

2578-2607 


280 

7250-7580 


88 

2607-2638 


200 

7580-7950 

7606-729 

97 

2638-2671 

2652-55 

550 

7950-8350 


101 

2671-2705 

2697-701 

182 

8350-9040 


168 

2705-2741 


93 

9040-9710 


183 

2741-2780 

2753 

135 

9710-10480 

10140 

580 

2780-2820 

2800-07 

310 

10480-11360 

11187-290 

340 

2820-2861 


84 

11360-12280 

11890-2130 

280 

2861-2904 

2894 

170 

12280-13200 


167 

2904-2949 

2925 

82 

13200-14100 

13570-955 

400 

2949-2998 

2967 

460 

14100-14980 


133 

2998-3050 

3021-27 

730 

14980-15850 

15300 

174 

3050-3106 


91 

15850-16700 


118 

3106 3165 

3126-32 

1,200 

16700-17520 

16900,17100 

330 

3165-3226 


93 

17520-18300 


106 

3226-3291 

.... 

51 

18300-19040 


81 

3291-3360 

3341 

200 

19040-19740 


75 

3360-3434 


55 

19740-20420 


69 

3434-3514 

.... 

35 

20420-21080 


61 

3514-3601 

«... 

55 

21080-21720 

.... 

57 

3601-3696 

3650-63 

1,900 

21720-22340 

.... 

50 

3696-3798 

.... 

170 

22340-22970 


50 

3798-3909 

3902-3906 

100 

22970-23590 


52 

3909-3960 


34 

23590-24200 


42 

3960-4019 


43 

24200-24800 

.... 

39 

4019-4079 

4047-78 

550 

24800-25390 


39 

4079-4142 

.... 

82 

25390-25960 

.... 

36 

4142-4209 


34 

25960-26530 


29 

4209-4280 


45 

26530 27080 


27 

4280-4431 

4339-58 

1,140 





90 


SURVEY OF INFRARED SOURCES 


Table 11. Spectral distribution of radiation from 
4-watt germicidal lamp.* 


Wavelength 

Angstroms 

Total output 
milliwatts 

Wavelength 

Angstroms 

Total output 
milliwatts 

2436 

0.40 

3255 

0.15 

2460 

0.37 

3322 

0.34 

2485 

0.36 

3394 

0.19 

2511 

1.6 

3472 

0.22 

2537 

600 

3556 

0.26 

2564 

1.1 

3648 

9.0 

2592 

0.25 

3745 

0.40 

2622 

0.0 

3852 

0.0 

2655 

0.86 

3931 

0.55 

2689 

0.0 

4049 

11.4 

2725 

0.0 

4175 

0.42 

2760 

0.17 

4358 

25.6 

2800 

0.16 

4560 

0.19 

2840 

0.16 

4742 

0.16 

2881 

0.59 

4947 

0.21 

2925 

0.14 

5188 

0.33 

2972 

1.9 

5461 

15.1 

3022 

1.2 

5773 

3.9 

3075 

0.0 

6143 

0.26 

3131 

9.0 

6587 

0.20 

3192 

0.14 

7105 

0.24 


*Based on rated output of 0.6 watt, 2537 A at 100 hours life. Initial outputs 
will be approximately 25 per cent higher. The average output throughout 
life is approximately 0.5 watt at 2537 A. 


to the axis of the arc tube. This brightness for the A- 
H4 lamp (which uses the same basic 100-watt element 
as the C-H4, E-H4, and J-H4) is 8 candles per square 
millimeter. For the three PAR lamps using this mer- 
cury element, the brightness would be somewhat less, 
and its spatial distribution would depend on the ac- 
curacy of positioning the arc tube in the reflector and 
the type of cover glass employed. Figures 34 and 35 
give the candlepower distribution for typical C-H4 
and F-H4 lamps. 

The brightness of the 1,000-watt water-cooled A-H6 
is approximately 300 candles per square millimeter 
and the A-HIO and B-HIO have element brightnesses 


Table 12. A-HlO beam output. 


Beam candlepower 

With reflector 

Without reflector 

7-inch 

490,000 

2,070 

8-inch 

509,000 

2,360 

Beam spread to 
half maximum 

Vertical 

Horizontal 

7-inch 

1°42' 

6°6' 

8-inch 

2°42' 

7°29' 


of approximately 65 candles per square millimeter. 
As in the case of the C-H4 and F-H4, the unit (com- 
plete lamp) brightness is lower because of the reflec- 
tion factor of the built-in reflector. The candlepower 
distribution curve of an A-ITIO lamp is given in 
Figure 36. It will be noted that the spread to half- 


maximum is somewhat different than obtained by tests 
at Rochester (see Table 12). 

Beam Output and Distribution In Specific Optics. 
The distribution curves (Figures 34 to 36) indicate 
the maximum unfiltered candlepower for all the 
projector- type mercury sources covered in this report 
except the J-H4. The latter was employed by the In- 
stitute of Optics, University of Rochester for UV 
night landing. Experimental lamps using the H-4 



UP OR LEFT DOWN OR RIGHT 


DEGREES 

Figure 36. Percentage candlepower distribution for 
the 7- inch 400- watt A-HIO mercury lamp, maximum 
candlepower 600,000. 

mercury element were mounted both horizontally and 
in the end-on position within the parabolic reflector. 
No data on outputs are available. 

The A-II6 lamp employed in the sea-search project 
(Section 9.5) had a candlepower of 5,300 with 120 
volts on the primary of the transformer. In the 24- 
inch parabola (10-inch focus, reflection factor 0.91, 
arc area 0.25 square centimeter), the beam candle- 
power was 52,700,000, with beam spread of % degree 
to half-maximum. 

The A-HlO UV source gave the outputs and dis- 
tribution at the Institute of Optics shown in Table 12. 

Filters. For ultraviolet uses, the Alters shown in 
Table 13 were employed. 


Table 13. UV filters with mercury-vapor lamps. 


Source 

Filter 

J-H4 

UV filter 3 mm #5874 plus 

3 mm #5860 

A-HIO, B-HlO 

UV filter 3 mm #587 plus 

3 mm #586 

4-watt germicidal lamp 

UV filter 1.78 (10 mm) molar N-Cl 
and 5.3 mm of Corning # 9863 
(Wavelengths transmitted, 2537 A 
group and 3130 A group — 90 
per cent in 2537 A group). 


RFi^THTiiiriiii^ > 


Chapter 6 


ULTRAVIOLET SOURCES AND FILTERS 

By Charles A, Federer, Jr.^ 


6.1 INTRODUCTION 

I N THE COURSE OF NDRC investigations of optical 
methods of military communication and recogni- 
tion, ultraviolet [UV] radiation has been given con- 
siderable attention, for under certain circumstances 
it furnishes relatively high security and satisfactory 
ranges. Chief contractors for this work have been the 
University of California (Department of Physics), 
OEMsr-1073; the University of Rochester (Institute 
of Optics), OEMsr-1219; and the New Jersey Zinc 
Company, OEMsr-740. 

Perhaps the chief contribution to the progress of 
UV communication has been the invention of the 
gallium lamp, described in Section 6.2.3, as a result of 
work at the University of California. This lamp pro- 
duces radiation of wavelengths invisible to the scotopic 
(dark-adapted) eye, thereby overcoming the objection 
to such sources as the high-pressure mercury arc, 
which can be seen several miles away on a dark night, 
even though the source is well-filtered. 

Although developments in optical communication 
by infrared means have outstripped those in the ultra- 
violet, principally because of the objection to ultra- 
violet visibility just mentioned, UV can often be used 
by day when infrared is impractical. 

Regions of the Ultraviolet 

The ultraviolet may be considered as extending from 
0.4 micron to 0.3 for the near ultraviolet [NUV], 
from 0.3 to 0.2 for the middle ultraviolet [MUV], 
and shorter than 0.2 micron for the far ultraviolet 
[FUV]. The NUV can pass through the eye lens and 
cause fluorescence of the retina ; the FUV suffers rapid 
atmospheric extinction ; consequently,, only the MUV 
is practical for military security. Even at 0.2 micron, 
however, the atmospheric transparency is practically 
zero, and the most useful portion of the MUV is from 
0.3 to 0.25 micron. 

The region of the MUV most suitable for military 
use is further narrowed by the factors illustrated in 

^Harvard College Observatory. The material in this chapter 
has been prepared principally from the report^ on ultraviolet 
communication written by Dr. Harvey White, of the Depart- 
ment of Physics, University of California at Berkeley. 


Figure 1, so that a desirable UV source is one which 
emits strongly and efficiently in the region 0.270 to 
0.295 micron, with as little radiation as possible at 
longer wavelengths. 

Ranges of the Ultraviolet 

On an aA^erage clear day, the atmospheric transmis- 
sion for the visible region of the spectrum is about 60 
per cent per mile. At this same time, in the 0.270-0.295- 
micron band, the transmission is about 40 per cent 



Figure 1. Graphs of factors affecting transmission 
and operation in the ultraviolet region. 


per mile, and this latter is taken arbitrarily as the 
practical value to be expected under this condition. 
If Rq is defined as the maximum distance in miles 
between light source and receiver at which satisfactory 
operation could be maintained if atmospheric trans- 
mission were 100 per cent, R is the actual range in 
miles of a source and receiver, and T is the atmos- 
pheric transmission per mile, then: R = Rq(T)^^^. 

Figure 2 is a graph of this equation, and shows 
clearly the ranges to be expected in the field. The 
values of Rq have been determined from laboratory 
measurements in Avhich the light path is through only 
a few feet of air. For example, the curve labeled Rq = 
20 miles is used for a source-receiver system which 
has a vacuum range of 20 miles, as measured in the 
laboratory. On an average clear day when the visibility 
range is 10 miles, the UV transmission T is taken 
as 40 per cent per mile; the practical field range is 
3.6 miles, or about 6,300 yards. 




91 


92 


ULTRAVIOLET SOURCES AND FILTERS 



Figure 2. Graphs giving actual range R to be expected in the field, as they depend upon laboratory-measured vacuum 
range Rq and the existing atmospheric transmission T. 


Table 1. Field ranges attained with invisible ultraviolet sources of radiation. 
(Values are given in yards and are limiting ranges determined for X = 0.280)U and 
an atmospheric transmission of 40 per cent per mile.) 


Source 

Receiver 

High- 

pressure 

mercury 

arc 

400 Avatt 
4°xl0° 
beam 

Low- 

pressure 

mercury 

arc 

25 watt 
360° 
beam 

Gallium 

lamp 

50 watt 
15°x25° 
beam 

Magnesium 

spark 

250 watt 
6°x6° 
beam 

Carbon 
arc d-c 

65 amp 
2°x2° 
beam 

Carbon 
arc d-c 

65 amp 
grid- 

modulated 

6°x6° 

beam 

Flame 

arc 

50 amp 
500 c 
360° 
beam 

Phosphor metascope for 
visual perception 

Day 



200 





Night 

8,500 


2,000 

>2,000 




Autocollimators seen 
with unaided eyes 

Day 








Night 

3,500 


750 

1,000 




1,000-c source signal and 

2°x2° photomultiplier receiver 

Day 


1,600 


5,000 

11,000 


6,200 

Night 








2°x2° photomulti[)lier 
voice receiver 

Day 


800 

800 



4,500 


Night 



5,000 



>9,000 


()uartz triple-chojiped return 
and multiplier receiver 

Day 





>1.400 



Night 








Geiger-Mueller tube 
counter 10° receiver 

Day 



200 





Night 



»1,000 





(Quartz triple return 
and metascope 

Day 



30 





Night 



200 






ULTRAVIOLET SOURCES 


93 


Table 1 gives the practieal ranges, R, of various 
ultraviolet sources with ditferent receivers. Spaces not 
filled in are not considered practical either because of 
visual insecurity or shoitness of range. 

UlTEA VIOLET FlUOEESCENCE 

More substances fluoresce under NUV radiation 
than under MUV, but some substances fluoresce under 
the shorter wavelengths and not at all under the 
longer, and vice versa. The spectacles of an unknowing 
observer in the path of UV light may fluoresce, and 
similar action within the eye itself results in the 
visibility of ultraviolet sources at great distances. The 
eye lens is opaque, however, to MUV radiation. Binoc- 
ulars of ordinary glass are also opaque to UV. 

The fluorescence with spectacles led to the testing 
of the coverings of aircraft taken from captured en- 
emy planes. It was hoped that the plastics used in the 
cabin cowlings of such planes would fluoresce under 
ultraviolet light, thereby cutting off the piloTs vision 
through to the outside. Many samples were tried, 
with disappointing results because the fluorescence is 
far too weak to have any significant effect. 

611 Results of NDRC Studies 

The following are the principal results of work by 
NDItC on ultraviolet recognition and communication 
systems. 

1. The high-pressure mercury-arc source was stud- 
ied and its use abandoned because as a source it was 
not entirely invisible, and some of its surroundings 
gave visible fluorescence (see Section 6.2.1). 

2. Two new types of UV sources, the gallium lamp 
(see Section 6.2.3) and the magnesium spark lamp 
(see Section 6.2.4), were developed to meet the secur- 
ity requirements for night operation. Both lamps give 
usalde ranges when employed as identification mark- 
ers or for communication with International Morse 
Code. 

3. The gallium lamp can be voice-modulated and 
speech-transmitted through it with 100 per cent intel- 
ligibility for 5,000 yards on an average clear night. 

4. Carbon arcs can be used as sources of ultraviolet 
radiation in the daytime, with usable ranges for both 
signal and voice communication. A grid-modulated 
carbon-arc source has been made that gives good voice 
communication at 4,500 yards by day. 

5. A flame arc, without any reflector, gives 360- 
degree coverage, and can be picked up in the daytime 


at 6,000 yards. This source could be voice-modulated 
for a 3,500-yard range on an average clear day. 

6. Table 1 gives the field ranges attained in tests 
with the UV systems developed by NDRC. 

62 ULTRAVIOLET SOURCES 

6 2.1 High-Pressure Mercury Arcs 

The high-pressure mercury arc was invented some 
3 ^ears ago by Bol, and is considered one of the strong- 
est of light sources, its intrinsic surface brightness 
being comparable to that of the sun. The radiation 
of this lamp is very high in the NUV, but relatively 
weak in the MUV. Most systems employing mercury 
arcs for UV radiation use part of the NUV, as de- 
scribed below. If only the MUV is used, the radiation 
is weak and very small ranges are obtained. 

Nevertheless, in the fall of 1941 and the first half 
of 1942, extensive experiments were performed under 
NDRC Contract OEMsr-1219, in which various sizes 
and styles of these arcs were studied. The most effec- 
tive unit developed employed a 400-watt air-cooled 
GE mercury arc mounted at the focus of a 6-inch 
parabolic mirror and equipped with either one of 
two filters. 

With the Corex 9863 filter (see Section 6.4.1), the 
above source showed no visible light, but wavelengths 
0.366 micron and 0.313 micron were strongly emitted. 
With the nickel filter (see Section 6.4.2), 0.366 mi- 
cron was cut off, and only 0.313 was visible, but even 
this caused significant fluorescence in the naked eye 
of an observer a mile away. 

On an average clear night, these sources can be read- 
ily located and observed by means of an ultraviolet 
phosphor metascope (see Chapter 3) at 5 or 10 miles. 
Field tests with autocollimators (see Chapter 7), set 
up at distant points and viewed with the unaided eyes 
of the observer at the source, gave ranges almost iden- 
tical with the visibility of the source itself to the un- 
obstructed eye, 2 miles with filter 9863 and 1 mile 
with the nickel filter. • 

A number of successful airborne tests were per- 
formed with high-pressure mercury-arc sources on a 
plane and with rows of autocollimators lining the run- 
way of a landing field. The runway was first seen at 
about 2 miles; good approaches and landings were 
made on all trials. The general results of various other 
field tests were all satisfactory, but the lack of secur- 
ity ruled out further development. 


94 


ULTRAVIOLET SOURCES AND FILTERS 


For daytime use, although the mercury arc gives a 
strong NUV radiation, sunlight and sky radiation 
are so strong that receivers covering any appreciable 
angular field are overloaded. The MUV radiation is 
too weak for use by day, so on this count, also, the 
high-pressure mercury arc is ruled out. 

^ Low-Pressure Mercury Arcs 

On numerous occasions, low-pressure mercury dis- 
charge tubes have been tested as sources of ultraviolet, 
using their strong radiation at 0.2537 micron. (High- 
pressure mercury arcs do not emit this wavelength 
because such light is strongly absorbed by the sur- 
rounding mercury vapor.) 

While the efficiency of the low-pressure mercury-arc 
discharge is higher for 0.2537 micron than is the 
gallium lamp for 0.290 micron, the atmospheric at- 
tenuation is considerably greater. As a consequence, 
the ranges obtained on average clear days are com- 
parable to gallium, and are therefore inadequate for 
daytime use. Since mercury tubes are widely manu- 
factured in long straight designs, it appears feasible 
to mount a considerable number of tubes in a cylinder 


arc spectrum offered interesting possibilities as a 
source of invisible ultraviolet radiation. Preliminary 
attempts to substitute gallium for mercury in com- 
mercial quartz lamps failed until further effort was 
expended in the purification of the gallium. A lamp 
consisting of a pool of gallium with a tungsten anode 
and filled with argon operated successfully but with 
low brilliance even at dull-red heat. Another attempt 
to substitute gallium for mercury in the Bol high- 
pressure mercury arc produced a very bright source 
but the life was only long enough to obtain spectro- 
grams showing typical gallium lines. 

Other experiments with potassium chloride, gal- 
lium chloride, bromide, and finally iodide, showed that 
it was necessary to use one of the volatile compounds 
of gallium to obtain reasonable brightness at conve- 
niently controlled temperatures. Only gallium iodide 
was free of objectionable lines in the NUV region. 
(At the time of this development, optical filters iso- 
lating the NIIV from the MUV were not considered 
practical.) 

Table 2 gives the spectral distribution of the gal- 
lium iodide discharge lamp, as determined from one 


Table 2. Emission spectrum of the gallium lamp (microns). 


Wavelength 

Milliwatts 

Wavelength 

Milliwatts 

Wavelength 

Milliwatts 

Wavelength 

Milliwatts 

band 

steradian 

band 

steradian 

band 

steradian 

band 

steradian 


.2448-2472 

0.3 

.2705-.2741 

0.07 

.3696-.3798 

0.02 

.5232-.5512 

0.07 

.2472- .2498 

0.1 

.2741-.2854 

0.0 

.3798-.3909 

1.2 

.5512-5835 

0.02 

.2498-.2524 

0.5 

.2854-.2897 

4.7 



.5835-6215 

0.0 

.2524-.2550 

1.5 

.2897-2921 

0.2 

.3876-.3989 

6.1 

.6215-.6672 

0.07 

.2550-.2607 

0.0 

.2921-2968 

9.1 

.3989-4110 

19.1 

.6672-.7210 

0.05 

.2607-.2638 

0.8 

.2968-3018 

0.03 

.4110-4244 

18.1 



.2638-2671 

0.5 

.3018-.3601 

0.0 

.4244-.4391 

0.08 

.7105-.7760 

0.2 

.2671-2705 

0.04 

.3601-.3696 

0.07 

.4391-5232 

0.0 

.7760-.8545 

0.6 







.8545-.9415 

0.5 


and to obtain a 360-degree distribution with consider- 
able range. Voice modulation of the tubes is not diffi- 
cult, so that they could be used for general voice 
communication. 

It is difficult to filter properly the low-pressure 
mercury arcs for nighttime security. This is due large- 
ly to the emission of light in the NUV. The gallium 
lamp does not present this problem and is therefore 
considerably more effective for night work. 

The Gallium Lamp 

Intkoduction 

A systematic survey of the spectra of all the chem- 
ical elements led to the conclusion that the gallium- 


lamp sent to the General Electric Lamp Works, Nela 
Park, Contract OEMsr-423. 

Because a filter must be used over the receiver photo- 
tube or the metascope to cut out solar radiation, day- 
time ranges are only one-third as great as those ob- 
tained at night, and daytime use of the gallium lamp 
is not promising. 

Compared with the high-pressure mercury arc, this 
lamp is very easy on the eyes. No ill effects have been 
experienced by any workers operating for many hours 
day after day. The peak gallium-lamp radiation is 
absorbed by the cornea of the eye and never reaches 
the retina. 


ULTRAVIOLET SOURCES 


95 


Lamp Construction 

Details of the construction of the gallium iodide 
lamp are given in the contractor’s reports After pre- 
liminary work with lamps of different sizes and 
shapes, a final design was developed to be used with a 
G-inch reflector for portable service (Type 6GS6). 
The discharge tube itself is shown in Figure 3, lamp 
and reflector in Figure 4, and portable assembly in 
Figure 5. 

The Discharge Tube. This f used-quartz tube is held 
mechanically in a f used-quartz jacket. The tube is 
made by bending a suitable length of 6-millimeter 
fused-quartz tubing into the shape of a U. The fin- 
ished U-tube is about 7 centimeters long and 1.7 cen- 
timeters wide. An oxide-coated tungsten wire is wound 
into a close helix 3 to 4 millimeters long and 1 milli- 
meter in diameter and is sealed at the end of each 




Figure 3. Gallium lamp. Upper — Discharge tube 
assembly. Lower — Disassembled unit showing dis- 
charge tube, thermostat and jacket. 



Figure 4. Gallium lamp and reflector. Upper— 
Rear view with housing removed to show electrical 
parts and assembly. Lower — Front view with filter 
removed to show mounting of discharge tube. 


leg of the IT. Each electrode is separated from the 
main discharge space by a 1-millimeter section of 
fused-quartz capillary selected to fit into the 6-milliF 
meter tubing. A seal-off tube is provided at the bend, 
and used for evacuating and finally filling the tube 
with pure gallium iodide and a mixture of gas con- 
sisting of 20 millimeters of argon and 5 millimeters 
of neon. This tip serves finally as a reservoir for the 
gallium iodide and is thrust through a metal shield 
so as to be in immediate contact with a bimetallic 
thermoregnlator. 





96 


ULTRAVIOLET SOURCES AND FILTERS 



Figure 5. Gallium lamp portable assembly. Upper 
— Assembled, complete with batteries and carrying 
stand. Lamp and one battery when removed form one 
complete unit for code communication. Spare battery 
is for prolonged operation. Lower — Disassembled unit 
showing component parts. 

The gallium was purified by several chemical meth- 
ods, all of which proved wasteful or time-consum- 
ing. The method of purification finally adopted con- 
sisted first of sealing a sample of the metal in a fused- 
quartz tube, evacuating and heating so as to distill off 
the volatile constituents and leave the gallium plus 
nonvolatile constituents behind. The distillation was 
performed at red heat (650 C) and the residue cooled 
with solid carbon dioxide and then transferred to a 
solution of dilute sulfuric acid (about 0.1 to 0.01 M). 
The acid was warmed until the gallium melted; then 


ice was used to cool the solution gently while at the 
same time a platinum wire dipped alternately into the 
pool of liquid metal and then into a crack in a piece 
of ice. This process brought about a slow growth of 
gallium crystals. 

When completed, the mass of crystals was made the 
anode of an electrolytic cell using a platinum cathode 
in the same acid solution. A"ery low current accom- 
plished the electrolysis, and the surface of the crystal 
became etched or tarnished. The process of recrystal- 
lization was repeated several times until the gallium 
no longer tarnished in the electrolysis. The final prod- 
uct was so pure that concentrated nitric acid did not 
attack it. Several methods of preparing small individ- 
ual crystals to be used in lamp making were developed. 
Individual vials of gallium iodide 2 to 3 millimeters 
in diameter and 10 to 20 millimeters long were also 
piepared. Each had a fragile capillary tip which 
could be broken by shaking the vial about in a larger 
tube containing a short section of rod to serve as a 
hammer. 

Filling and Stabilizing the Discharge Tube. Two 
lamps were sealed to a fused-quartz tube containing 
one of the vials and the mechanism for breaking it. 
The system was evacuated, thoroughly heated, and 
then filled with argon and operated while still on the 
vacuum line. The argon was changed several times 
until the electrodes formed and the argon showed the 
proper pinkish color. The final gas mixture of 20 milli- 
meters of argon and 5 millimeters of neon was then 
introduced into the system and the tube containing 
the vial of gallium iodide, together with the lamps, 
was sealed off the line. The vial was then broken and 
the gallium iodide distilled into the lamps. The lamps 
were again operated by using a neon-sign transformer 
at a current of about 30 milliamperes. When each tube 
seemed to reach the proper operating conditions, it 
was sealed off individually and the tube containing 
the vial was cleaned and made ready to be used for 
another lamp. 

If the operating temperature of the tube is too low, 
no gallium lines appear; if the tube is too hot, an ex- 
tremely high voltage is required to produce any dis- 
charge at all. For efficient operation, the temperature 
gradient in the tube must be carefully adjusted so that 
the solid gallium iodide collects and remains in the 
small cool pocket made by the seal-off tube. Before 
operating the lamp, it is necessary to distill most of 
the gallium iodide into this position by heat alone. 
The tube is then connected to the terminals of a cur- 
rent-limiting transformer capable of giving 40 or 50 


ULTRAVIOLET SOURCES 


97 


milliamperes at a voltage sufficient to operate the tube. 

The operating voltage is a function of the tempera- 
ture and of the condition of the electrodes. Individual 
lamps may vary in operation from 250 to 450 volts. 
A higher voltage means the lamp is either too hot or 
that all of the gallium iodide has not been distilled 
away from the electrode region. Newly made tubes 
should operate at about 40 or 50 milliamperes at the 
start, and then at betAveen 25 to 30 milliamperes. 
Should the lamp fail to stabilize, it is often necessary 
to turn olf the current and use heat alone to distill 
more gallium iodide. 

The Portable Lamp Unit (6GS6) 

This, unit is illustrated in Figure 5, and the elec- 
trical circuit for operation from a 6-volt storage bat- 
tery is given in Figure 6. This is essentially a con- 
ventional vibrator power-pack assembly using a spe- 
cial transformer with an open-circuit voltage of about 
1,500 volts. The function of the resistor inserted 
into the primary circuit is to limit the operating cur- 
rent through the lamp to about 25 milliamperes. 

Figure 7 shows the electrical circuit for modulating 
the gallium lamp for voice communication. The rec- 
tifier tube in the secondary circuit is important to 



Figure 6. Circuit diagram for portable gallium lamp 
(6GS6). R, 6-volt storage battery; Ci and Ca, 0.5- /if 
paper condensers; D, discharge tube; E, thermo-regu- 
lating element; L, radio frequency choke; Ri, 0.35-ohm 
coil of heavy resistance wire mounted in the lamp re- 
flector; R 2 , 1.2-ohm heating element inside jacket; /? 3 , 
50-ohm resistor; Si, toggle switch for “stand-by” con- 
dition; S 2 , signaling trigger switch; T, transformer, 
Gardner No. 4600; V, Radiart vibrator No. 5334. 

prevent distortion due to frequency doubling. This 
circuit, adapted from existing equipment, is not neces- 
sarily the best or the most efficient arrangement. Field 
tests show no difficulty in obtaining 100 per cent 
intelligibility. 


The entire portable source housing, 6 inches in 
diameter and weighing 6 pounds, is mounted between 
the uprights of a frame carrying tAVo 6-volt, light- 
Aveight storage batteries. The complete unit, with all 
necessary gear, Aveighs 32 pounds. 

Operation and Visibility of the Portable Unit. As 
a marker on the ground, or carried and hand-oper- 



Figure 7. Circuit diagram for speech modulation of 
the portable gallium lamp (6GS6). Bi and B 2 , 6- volt 
batteries; D, discharge tube; E, thermo-regulating 
element; K, rectifier tube HK24-K; R 2 , heating ele- 
ment inside jacket; R 3 , 50-ohm resistor. 

ated for signaling, the unit can maintain continuous 
operation for 2 hours. The beam angle is 25 degrees 
horizontally and 15 degrees vertically, considering 
the limits at half the intensity of the central maxi- 
mum. On moonless nights, the unaided eye cannot see 
the radiating source beyond 5 to 10 yards. The ground 
and objects in front of the lamp fluoresce very faintly, 
but are not visible beyond some several yards. 

A 2-inch and a 5-inch metascope (see Chapter 3) 
with UV phosphor buttons and corrector plates that 
will transmit UV have been used to observe the gal- 
lium source. At 1 mile, it is easily seen with either 
instrument; 2 miles is probably the limit on average 
clear nights (transmission 40 per cent per mile). With 
phosphor metascopes equipped with corrector plates 
made especially for UV, twice this range can be ex- 
pected, but none have been made. Hand-held image 
tubes could be made that should enable these lamps to 
be seen at from 3 to 5 miles on average clear nights. 
It is evident that the gallium lamp development 
brings UV more nearly on a par with infrared as a 
means for night signaling Avith high military security. 

As a Sifjnaling Lamp. The lamp and one light- 
weight battery can be hand-held for signaling, a trig- 
ger on the lamp handle being used for coding. This 
unit has a weight of 17 pounds, will operate on code 


98 


ULTRAVIOLET SOURCES AND FILTERS 


for IV 2 hours, and has the same range as given above 
for continuous beacons. 

Voice Transmission. By directly modulating the 
current input to the discharge tube, voice transmis- 
sion is readily accomplished. A newly developed pho- 
tomultiplier tube (RCA 1P28 — see Chapter 2) was 
mounted at the focus of a 10-inch f/1 mirror and used 
as a receiver. On an average clear night, the voice 
range of this combination, with 100 per cent intelli- 
gibility, is about 3 miles (vacuum range 15 miles). 
Several gallium discharge tubes in a 12-inch reflector 
should give a 25 per cent increase in range with an 
equally wide beam angle. 

Use with Autocollimators. Five-inch W autocol- 
limators were made by BuAer for use with gallium 
lamps. At 600 yards, these markers accept UV radia- 
tion and return it as visible light to a circular fleid 
about 2 feet in radius around the source as a center. 
Thus, an observer within 2 feet of the source sees the 
returned beam, whereas it is otlierwise invisible. The 
autocollimators cover a held of about -15 degrees hor- 
izontally and 30 degrees vertically. 

The range for a single autocollimator is 600 yards, 
but a cluster of three of them can be seen at 750 yards 
in average clear weather. Twice this range is obtained 
with 7x50 binoculars, and 10 per cent greater range 
when single or triple clusters are lined up in rows, 
as they were when used to line the runway of a land- 
ing fleid. 

The Magnesium Spark 
Inteoduction 

The magnesium spark involves essentially an elec- 
trical spark discharge from condensers between mag- 
nesium electrodes operating on high voltage direct or 
alternating current. The UV radiation consists of four 
very strong lines from ionized magnesium atoms. 
Their wavelengths, 0.2791, 0.2796, 0.2798, aird 0.2803 
micron, lie exactly in the more desirable region of the 
spectrum. Radiation of very low intensity from neu- 
tral magnesium is also present at wavelengths 0.2852, 
0.3097, 0.3337, and 0.3838, but if the potential across 
the spark gap is high and the spark is quickly 
quenched, this neutral radiation can be kept insig- 
nificant. 

The spark itself is intensely bright, measures about 
inch, and gives a fairly narrow beam when 
mounted in a 6-inch parabolic reflector. It cannot be 
modulated for voice, but does respond perfectly to 
code signaling. 


ElECTEICAL ClECUlTS 

For the magnesium source to be practical, the aux- 
iliary electrical equipment must not be too heavy, the 
radiation must be of high efficiency, and filters must 
suppress undesirable wavelengths of radiation. Sev- 
eral types of circuits have been found suitable, and 
these are described in the contractor’s report.^ 

One method employs high-frequency excitation by 
charging a condenser placed across the secondary of 
a transformer, and discharging it through a spark 
gap, as shown in the upper part of Figure 8. Spark 
lines, rather than arc lines, are produced as the volt- 
age drops very rapidly because of the high-frequency 
oscillation of the discharge. The number of sparks per 



TA ~ A UTO jRANsro^Men 
L - JOOwH. Choke Co/ls 
’S - Gap 

(A) (B) 

T - / K(/A -Z5000 Ifo/t- TRAN^FOnMEP - ^ MTi/A - QOOdvolt 
C' O.OOey>*.f - Z5 KU'CoAjnENSER " -/O 



3PAnK Gap 

^h/-SYNCHFONOUS SiaI/TCH 
/<£NCTR0N 
C- CcrWENSER 


^PAPK gap 
Cr o,ooz- aoo4 
Kr HK-Z4--K 
'R,- ZOvj FesiSTon 

T- GaRDMEP- JOOO¥olt. 


500 VA. 

400^-r><AHS FORMER 


Figure 8. Diagrams of circuits used with magnesium 
spark. 


half cycle depends on the gap length, capacity of con- 
denser, and charging voltage. It is found that the in- 
tensity of the spark lines is increased by increasing the 
breakdown voltage and the number of sparks per 
second, optimum values being chosen by trial. 

In a second method, the condenser across the spark 
gap may be charged continuously with high-voltage 
direct current, as shown in the lower left of Figure 


ULTRAVIOLET SOURCES 


99 


8. Resonant charging conditions are then absent, pro- 
viding complete control of the spark frequency by 
means of a synchronous gap. Very rapid rise and fall 
of the voltage results in enhancement of spark-line 
intensities, but the apparatus is bulky. 

The first method mentioned above is difficult to use 
with power-line frequencies higher than 60 cycles since 
the condenser reactance becomes so low. To obviate this 
difficulty, the rectifier circuit shown at the lower right 
in Figure 8 was devised. Two spark gaps operate in 
parallel, one on one half of the cycle and the other 
on the other half. The transformer has a filament 
winding at each end of the secondary for heating 
the kenotron cathodes. This arrangement has been 
found quite suitable for operation from a 0.5-kilo- 
volt-ampere, 400-cycle airplane inverter, and could 
presumably be operated just as well from an 800-cycle 
one. 

SparTc Intemities. In order to increase the sharp- 
ness of gap breakdown, various types of series mul- 
tiple gaps were tried; they all enhanced the spark 
lines and might well be adapted to certain applica- 
tions. Gap length and atmospheric pressure also in- 
crease the voltage at breakdown. The intensity of the 
0.28-micron emission was determined as a function 
of the gas atmosphere in which it occurred. Atmos- 
pheres of air, nitrogen, hydrogen, helium, argon, and 
carbon dioxide were tried; argon appeared best, but 
impractical. Air works quite well, although carbon 
dioxide is perhaps a little better and might be practical 
in some cases. 

To prevent the sputtering of magnesium on the 
reflector, it is necessary that the gap be surrounded by 
a f used-quartz jacket in which there is a rapid flow 
of gas. This is readily accomplished by using a high- 
pressure atmosphere in the jacket with a small amount 
continuously bled out. Bleeder holes in the electrode 
centers resulted in more rapid quenching. A satisfac- 
tory spark and jacket design is shown in Figure 10. 

Lajige Magnesium Soukce 

The lamp in Figure 9 is the large source usually 
mounted on a tripod for field operations. Figure 10 
shows the construction and design of this lamp. A 
vacuum-cleaner unit is used with it to pull air through 
the spark and remove the sputtered magnesium pow- 
der from the quartz jacket. 

All field tests on which ranges for this and the 
portable source have been quoted were carried out 
with a single spark gap mounted coaxially in a para- 
bolic glass reflector. In the case of the 1-kilovolt-am- 



Figure 9. Magnesium lamp. Upper — Front view of 
large lamp with filters removed. Lower — Special fused- 
quartz jacket for magnesium spark. 


pere source, this reflector is 6 inches in diameter by 
1-inch focal length and front aluminized. Some six 
field tests were made with the large source, using 5- 
in(;h autocollimators and a 5-inch metascope as re- 
ceivers. With a 6 by 6 degree beam and 250-watt 
circuit, a range of 2,000 yards was achieved with 
phosphor metascopes. With a photomultiplier tube 
as a receiver, the range may be increased to about 
4,400 yards (see '‘Radar Principles'^ below). Auto- 
collimator ranges are about 1,000 yards in average 
clear weather. 

When this source is turned on, fluorescence of spec- 
tacle lenses is observed not farther than 50 yards from 
the source; at 440 yards, a slight fluorescence is dis- 
cernilfio, but an unknowing observer could not tell 
the source of the radiation. 


nrTii|^|T^ 




100 


ULTRAVIOLET SOURCES AND FILTERS 



Hand-Held Magnesium Source 

For a hand-held unit of about 60 volt-amperes 
capacity, the reflector is of 4 inches diameter and 1- 
inch focal length, the entire unit weighing less than 
4 pounds. The power pack, of about 15 pounds, 
contains a 25-ampere-hour, 6-volt storage battery, a 
small 2,000-volt step-up transformer, and a standard 
60-watt vibrator unit. 

With a suitable filter in front of the source, this 
lamp can be used on the darkest night Avithout being 
seen. Suitable electrical filtering is provided, par- 
ticularly for code signaling. The range for satisfactory 
communication on an average clear night is about 
1,000 yards. 

IiADAR Principles 

Due to the nature of the electrical discharge, the 
usable ultraviolet radiation is emitted in pulses of 
about 1 to 2 microseconds duration. Because of the 
consequent brightness of the pulses, the distance at 
which the signals can be picked up by a receiver with 
phototube and amplifier is very great. The pulse re- 
sponse of such a receiver depends on the pulse inten- 
sity and not upon the time average over a succession 
of pulses as is the case with any viewing device. 

IBRAD or radar principles'’ are therefore suggested 


as highly promising when applied day or night to the 
magnesium spark as a source of radiation. A f used- 
quartz triple prism used to return the light toward the 
source presents an excellent opportunity for many 
short-ranging problems. This problem was not inves- 
tigated to any extent. 

6.2.5 High-Intensity Carbon-Arc Sources 

Introduction 

In Chapter 5 these sources have been described in 
detail as to their general construction and operating 
characteristics. Carbon arcs cannot be used for security 
in night communications, because filters are not avail- 
able to remove their very strong NUV, but in the day- 
time they are secure, as far as visibility is concerned, 
and give sufficiently strong radiation in the MUV 
for practical use. 

National Carbon Company’s U-carbon was chosen 
as best for UV work. With 50 volts and 60-ampere cur- 
rent, such an arc is known as a ‘^ffiigh-intensity flame 
arc.” The increased brilliancy as compared with that 
of a low-intensity arc is produced by radiation from 
the flame materials within the confines of the arc 
center. 

The 24-inch Searchlight 

In the experiments here reported, these arcs were 
run in a Navy 24-inch searchlight similar to those 


•’See STR Division 16, Volume 3, Chapter 6. 


ULTRAVIOLET SOURCES 


101 


described in Chapter 5. In most cases, U-carbons were 
used as the rotating positive carbons. Slight modifica- 
tions in the automatic feed circuit were required for 
these carbons. With an 11-millimeter positive and an 
8-niillimeter negative carbon, the arc drew a current 
of 60 amperes with an arc drop of 38 volts. The usable 
UA" from this arc consists principally of two peaks 
between 0.25 and 0.29 micron ; filters must cut out all 
wavelengths longer than this. In some cases. Corex 
9863 glass, made up of 6-inch squares mounted in a 
cell-like structure, was used to cut out visible light. 
For complete security a G-filter, described later, 
should have been used, but there was no time to make 
one of suitable size. In the actual tests, inasmuch as 
the Corex filter affected the range only slightly, it was 
often not used at all. 

The beam from the searchlight was chopped by 
two slotted wheels to obtain 1,000 cycles per second 
to suit the electrical filter already available for the 
receiver. One of the wheels remained stationary while 
the other rotated. A frequency of 90 to 100 cycles is 
recommended, however, if mechanical chopping is 
used at the source. 

A receiver, consisting of an EC A photomultiplier 
tube (1P28), an ultraviolet light filter, a high-gain 
tube amplifier, a 1,000-cycle electrical filter, and ear- 
phones, was used. The phototube was mounted in a 
small, light-tight box and located at the focus of a 
10-inch diameter, 10-inch focus mirror, as shown in 
Figure 11. The field angle taken in by this receiver 
was approximately 1 degree horizontally by 4 degrees 
vertically. 

With the source mounted high on the top of a build- 
ing and the beam chopped at 1,000 cycles, the receiver 
was taken to various distant vantage points and the 
signal picked up by listening for the 1,000-cycle note 
in the earphones. On one exceptionally clear day, a 
strong signal was picked up at 13 miles. Since in the 
laboratory this source and receiver have a measured 
vacuum range Hq of 130 miles, the atmospheric trans- 
mission for that one test was the highest ever meas- 
ured by this group, about 70 per cent per mile. On an 
average clear day when the visibility is only 10 miles, 
the ultraviolet transmission of the atmosphere will be 
about 40 per cent per mile (T = 0.40), and the actual 
usable range of 6 to 7 miles will be achieved. 

With only the Corex filter in front of the source 
and the 2 by 2 degree searchlight beam turned on the 
receiver, observers at the receiver can see the source 
as a red light. Though a large G-filter was not avail- 
able at the time, such a filter can be made, and meas- 


urements already at hand show that the above ranges 
would not be decreased by more than 10 to 15 per 
cent by its use. 

The Triple-Peism Return 

Several f used-quartz triple prisms, 3 inches in aper- 
ture, were made by the Mount Wilson Observatory 



Figure 11. Photomultiplier receiver. Upper — Re- 
ceiver assembled, mounted on tripod for field use. 
Seven-inch collecting mirror. Amplifier and power sup- 
ply not shown. Lower — Receiver disassembled to 
show component parts. 


optical shop. The chopper was removed from the 24- 
inch searchlight, and the receiver phototube and mir- 
ror mounted at the center of the beam where the chop- 
per motor had previously stood. The triple prism was 



102 


ULTRAVIOLET SOURCES AND FILTERS 


carried to a distant vantage point and a chopper 
placed in front of it. On an average clear day, the 
return signal was picked up at 0.8 mile. Several prisms 
in a cluster should increase this range to 1 mile. It 
is worthy of note that in this system the source itself 
cannot be chopped without overloading the receiver 
with back scattered light. 

CURRENT-^IODULATED FlAME ArC 

To produce a daytime communication system with 
a 3G0-degree beam, large-sized special cored carbons 
(16 to 22 millimeters in diameter) were mounted ver- 
tically with no reflector. A 50-ampere current produces 
a conically shaped flame about 1 inch high and 1 inch 
wide at the top. With a Corex Alter, such a flame is in- 
visible to the unaided eye beyond 100 feet ; the Alter 
transmits about 70 per cent at 0.28 micron, and a voice 
range of at least 2 miles is assured on an average clear 
day. 

However, because of the large current required and 
the shortness of time available for development, a 
voice-modulation system was not used. In its place, 
a 500-cycle generator was used, causing the flame to 
dim at each half-cycle, or 1,000 times a second. 

This bare arc flame was picked up in the daytime 
at 3 V 2 miles by means of the photomultiplier tube al- 
ready mentioned and described below. This appears to 
be the usable range of the open flame arc when the 
UV transmission is 40 per cent per mile — a remark- 
able result when the 3G0-degree distribution of the 
radiation is considered. At the focus of a reflector re- 
stricting the beam to about 20x20 degrees, this source 
should give a range of from 5 to 7 miles. 

This system was not tested at night, birt the range 
curves and field measurements with other sources and 
receivers show that nearly twice the daytime ranges 
are assured. Proper filters should be fairly easy to 
produce because of the relatively low emission of vis- 
ible and NUV. 

Grid-Modulated Carbon Arc 

Figure 12 gives the circuit and optical system of a 
means for modulating radiation from a high-intensity 
flame arc for voice communication by day or night. 
'Idle system has complete visual security in the day- 
time, and might be made to have reasonable security 
at night. 

A 12-inch diameter elliptical mirror E focuses the 
light from the source S on the mirror which is 
spherical. The light passes through the uncoated strip 


spaces of the concave mirror il/^, made of fused 
quartz, with its radius of curvature twice the distance 
Mr to My. Thus mirror My focuses an image of the 
grid itself, and as My rocks back and forth, owing to 
voice modulation, the grid image moves back and 
forth, thus reflecting more or less light in a parallel 
beam into space. The beam is about 6 by 6 degrees, 
with fairly sharp edges. 

Modulation is accomplished by vibrating mirror My 
so that the grid lines move half their width. The 
grid mirror is 8 inches in diameter, and its strips of 
evaporated aluminum are of i^ch wide with 
spaces of equal width. The vibrating mirror My is 1^/2 
inches in diameter. 

In this system, a moving-picture projector is used 
as a source. It consists of a high-intensity automatic 
arc with reflector (Figure 13). With special U-carbons 
(9-millimeter positive carbon) a current of 60 am- 
peres is maintained with 50 volts across the arc. 
The standard ellipsoidal mirror is used to focus an 
image of the positive carbon on the vibrating mirror 
located at the position ordinarily occupied by the 
film gate. 

The vibrating mirror is attached to a coil in a 
magnetic field, somewhat like the arrangement of a 
ballistic galvanometer. The resonant frequency of the 
coil support is about 500 cycles per second, and damp- 
ing is provided by a small amount of silicone stopcock 
grease between the coil and the pole piece. The maxi- 
mum displacement of the vibrating mirror is about 
0.072 degree radian. The driving amplifier is of con- 
ventional design except that the coupling condensers 
are of much smaller capacity than usual ; this helps to 
give the mirror a constant response for all used audio 
frequencies. 

In principle, this system can be used to modulate 
any kind of radiation. It uses very little power for 
even the strongest sources; it is very compact and the 
optical system consists almost entirely of reflecting 
surfaces. 

Field tests were made with RCA photomultiplier 
tube receivers. These comprise a UV filter, a high- 
gain tube amplifier, an electrical filter with high- 
frequency cutoff, and earphones. The usable range by 
day was 2y2 to 3 miles, and twice this distance at 
night. The daytime range is lessened because of the 
need for a heavy light filter to shield the receiver from 
sky radiation by clay. 

Smaller vibrating-niirror elements used for sound- 
recording equipment could also be used for modulat- 
ing UV sources. 


ULTRAVIOLET SOURCES 


103 


10,000 ish 





Figure 12. Mechanical-optical modulator. Upjyer — Circuit diagram. Middle — Optical system. Lower — Voice coil and 
magnet design. 


104 


ULTRAVIOLET SOURCES AND FILTERS 



Figure 13. Voice-modulated carbon-arc transmitter. No filter is shown here. 


63 ULTRAVIOLET RECEIVERS AND 
AUTOCOLLIMATORS 

Metascopes 

Instruments of this general type are described in 
Chapter 3 of this volume, their use being chiefly for 
the detection of infrared radiation. Nevertheless, the 
first metascopes employed UV-sensitive phosphors. No 
UV metascopes have gone into the production stage, 
blit several types have been used in field tests of UV 
communication systems. 

In the UV metascope, radiation enters through a 
corrector plate, and after reflection by the spherical 
primary mirror is brought to a focus on a phosphor- 
coated spherical screen. The visible light emitted by 
the phosphor passes through the perforation in the 
primary mirror to be viewed by an eyepiece. Usually, 
the magnification is unity. 

UV phosphors do not require precharging as do 
those sensitive to infrared. However, it is believed that 
greater sensitivity can be obtained from UV-sensitive 


image tubes, but the construction of such instruments 
has only been contemplated. Future work on UV 
communication should give this proposal full consid- 
eration. Meanwhile, the most sensitive UV receivers 
have been photomultiplier tubes. 

6.3.2 Photoelectric Receivers 

Prior to the war, existing receivers for detecting 
UV below 0.3 micron were inadequate both in type 
and sensitivity, so it was decided to develop a photo- 
electric receiver of high sensitivity to modulated 
beams of such radiation. With modulated beams, the 
receivers could be used with fairly high background 
light (daytime), they could detect voice- and code- 
modulated radiation, and the detection would be aud- 
ible rather than visual. 

Each such receiver is an assembly of several com- 
ponents: (1) a suitably sensitive photocell; (2) a 
light collector; (3) an optical filter; (4) a suitable 
amplifier combined with an electrical filter for re- 
jecting unused energy-frequencies (noise). 


ULTRAVIOLET RECEIVERS AND AUTOCOLLIMATORS 


105 


As a result of the current amplification attainable 
with secondary electron emission, vacuum photocells 
extremely sensitive to UV have recently become avail- 
able. The RCA photomultiplier tube 1P28 amplifies 
the original photocurrent 10^ or more. This tube was 
used in spherical-mirror light collectors, one of 10-inch 
aperture and 20-inch focus, the other of 7-inch focal 
length and 7-inch diameter. The latter is more usable 
as the photocell has a sensitive surface, ^ inch wide 
by 1 inch long. Thus, the cell placed at the focus of 
the mirror has an angular aperture of 2 by 8 degrees, 
and by diaphragming this can be reduced to 2 by 2 
degrees or less. 

The amplifier is a 3-tube, battery-operated voltage 
amplifier designed to cut off frequencies below about 
600 cycles. It is combined with a low-pass filter hav- 
ing a cutoff at 3,000 cycles for receiving noise. The 
amplifier is coupled to a 1,000-henry choke in the 
anode circuit of the 1P28. Other methods of coupling 
may also be used. Dry batteries were used for con- 
venience. 

In case a single-frequency signal is used as a source 
of radiation, a band-pass filter having strong attenua- 
tion at all frequencies except the signal frequency is 
substituted for the low-pass filter mentioned above. 
This increases the signal-to-noise ratio. 

Figure 11 shows the complete receiver with the 7- 
inch mirror in place. In order to measure the range 
of this receiver with a 24-inch Navy searchlight, it 
was necessary to modulate the beam with the two 
slotted disks already mentioned. This signal was 
found to have a range of 22,000 yards in daylight on 
an unusually clear day, using the above described re- 
ceiver with the 10-inch refiector and a Navy-type 
1020-cycle band-pass filter. 

Other ranges achieved with photomultiplier tubes 
are shown in Table 1. 

Autocollimators 

As described in Chapter 7, autocollimators may be 
made with the usual Kellner-Schmidt system, and if 
the focal surface is coated with UV phosphors, the 
autocollimator will return visible light after illumina- 
tion by a UV source. Instruments of this type were 
developed by the University of Rochester in 1940- 
1941. 

If the corrector plate is corrected for spherical aber- 
ration in the UV, it is not corrected for the returning 
visible light, and vice versa. As a consequence, a com- 
promise had to be made in the 17 special 5-inch auto- 
collimators made by Navy BuAer for field tests by 


the University of California. This resulted in the 
visible return beam having a slightly increased spread, 
reducing the maximum range of each device but 
permitting mounting of the source several feet from 
the observer. 

As explained in Chapter 7, the intensity of the re- 
turned beam of an autocollimator is inversely propor- 
tional to the fourth power of the distance. When at- 
mospheric attenuation is taken into account, to double 
the limiting range of any one autocollimator-source 
system, the source beam must be intensified about 
100 times. 

6.3.4 Triple Mirrors of Fused Quartz 

Four f used-quartz triple mirrors (prisms), to be 
used as autocollimators for UV field tests by the Uni- 
versity of California, were made by the Mount Wilson 
Observatory. These prisms, with apertures of about 
3 V 2 inches, are in principle the same as glass triples 
commonly used with visible and infrared light (see 
Chapter 7). 

The manufacturing time for quartz is about the 
same as for glass, but inhomogeneity of the fused 
quartz results in a wider return beam than usual with 
ordinary glass triples. The quartz prisms return a 
beam within a cone of about 40 seconds of arc (1 foot 
in a mile), as against from 2 to 5 seconds of arc for 
visible light with optical glass. In some respects, how- 
ever, the wider return beam is an advantage. 

^ Photoglow Tubes 

An investigation of the use of copper-electrode 
photoglow tubes has established the fact that dis- 
charges can be initiated in neon at 25 millimeters 
pressure by very faint invisible ultraviolet radiation. 
It was determined from laboratory measurements that 
a 6-inch Kellner-Schmidt optical system, 1 mile away 
from the gallium lamp described in this report, would 
collect sufficient radiation to control these photoglow 
tubes. Furthermore, it was demonstrated by field 
measurements that such a neon discharge placed at 
the focal plane of a Kellner-Schmidt optical system 
is visible at a distance of over 1 mile. 

The actual work of developing a mosaic of photo- 
glow cells to replace the phosphor surface of the ultra- 
violet autocollimator has not been undertaken. How- 
ever, there is good evidence that such a mosaic could 
be perfected and that it would greatly increase the 
range of the present ultraviolet autocollimators. 

Before planning the actual investigation of a photo- 
glow mosaic, one alternative device seemed to deserve 


106 


ULTRAVIOLET SOURCES AND FILTERS 


investigation. This would be an electron image tube 
in which a relatively high potential is applied between 
two parallel screens no more than 1 millimeter apart. 
One screen should be a semitransparent photosurface, 
and the other a fluorescent surface. Photoelectrons, 
emitted by the ultraviolet-sensitive photosurface, 
should be drawn directly across the gap to impinge 
upon the fluorescent screen and give rise to a bright 
spot of visible light. Such an image tube, placed in the 
focal plane of a Kellner- Schmidt optical system, 
would make of it a longer-range ultraviolet autocol- 
limator. 

6.3.6 Geiger-Counter Receiver 

A copper Geiger counter, as described in published 
literature, was built and used in conjunction with a 
quenching circuit and an electronic relay. The tube 
was mounted at the focus of a 6-inch spherical re- 
flector having a focal length of 2^/2 inches. This unit 
was mounted on the top of a building with the relay 
connected to an ordinary light bulb for indicating to 
a distant observer the reception of a signal. 

A gallium lamp was carried at night to distant 
vantage points where the beam was directed back to- 
ward the receiver. Triggering the gallium lamp source 
as in code signaling caused the Geiger-counter receiver 
to operate and to turn on the white light. 

The tests showed that extremely faint ultraviolet 
radiation can be made to operate a relay. A night- 
time range with this first tube was found to be 1 mile. 
The daytime range is considerably less at the present 
time because of the filter that must be used around 
the counter tube. Further work on the chemical puri- 
fication of the copper surface and on the selection 
of other metals to replace copper shows promise of 
greater daytime ranges. 

The filter consisted of KAT (Section 6.4.4) in cel- 
lulose acetate. Since this detector is not sensitive to 
light in the visible, it is necessary to eliminate only 
the near ultraviolet from 0.31 micron up. This is 
accomplished sufficiently well by the NAT alone. 

6 4 FILTERS FOR ULTRAVIOLET 
SOURCES AND RECEIVERS 

Intkoduction 

Filters for UV play an important part in maintain- 
ing the visual security of signaling and communica- 
tion systems. The choice of a suitable filter depends 
on such factors as the relativity intensity of the emis- 


sion of the source in the NUV and the MUV, the use 
of modulated beams, and the distance at which com- 
plete security is required. 

In January 1945, it was first suggested that UV 
radiation might well be used in the daytime, and con- 
siderable effort was made to reinvestigate all UV 
sources. In daylight, the eye is not sensitive to the 
UV, and all filtering problems of a source are simpler, 
when compared to the problems for night security. 
Laboratory and field tests have shown that such 
sources as the gallium lamp and the magnesium spark, 
which work so well in darkness, are too weak to hope 
for any long daytime ranges. This is due principally 
to the relatively low ultraviolet transmission of the 
filter that must be placed in front of any receiver. 
A receiver filter must cut out all visible and near 
ultraviolet light down to 0.295 microns. Though ex- 
ceptionally good filters for this purpose have been 
made, their transmission in the desired region, 0.275- 
0.295 micron, is only 10 to 27 per cent at best. 
(See Figure 14, dot-dash curve labeled G -f- PDAA 
-f PDAB.) 

Fortunately, carbon arcs using carbons especially 
cored to give strong ultraviolet radiation of the de- 
sired wavelengths are very powerful sources, thereby 
compensating for the loss in sensitivity at the receiver. 

6 4 1 The Corex 9863 Filter 

This is Corning glass filter No. 9863, and has been 
found to be the best filter for cutting out most of the 
visible light emitted by an ultraviolet source. Its trans- 
mission curve is included in Figure 14 (long dashes) ; 
note its infrared transmission, and its opacity for 
radiation below 0.250 micron. 

Corex 9863 filters can be obtained only in sizes up 
to 6V2 inches square; both sides should be cloth- 
polished and 3 to 5 millimeters thick. Long exposure 
to sunlight or strong UV should be avoided, shields 
being used during daylight operation. 

6 4 2 The Nickel Filter 

This filter was developed to produce a semisolid 
filter that would not require a cell structure for sup- 
port, that would not leak (as may liquid filters), and 
would not easily break. Its principal absorbing agents 
are nickel chloride and nickel sulfate. Its absorption 
curve is shown by short dashes in Figure 14; note 
that it cuts off at 0.36 instead of 0.41 micron as does 
Corex. When combined with the Corex filter, the 
nickel filter becomes the G-filter, shown by the solid 


FILTERS FOR ULTRAVIOLET SOURCES AND RECEIVERS 


107 


curve in F'ig’iirc 14. Further details of the G-hlter 
are given in the next section. 

Prior to this NDRC project, the nearest that anyone 
had come to obtaining such a filter was a liquid filter 
containing nickel chloride or nickel sulfate in solution. 

6.4.3 G-Filter for the Gallium Lamp 

This filter consists of a sandwich of nickel sulfate- 
sorbitol complex between a plate of polished fused 
silica and a plate of Corex 9863. A thickness of 3.5 


gallium arc has virtually no radiation in this region. 

The nickel sulfate-sorbitol complex was developed in 
order to obtain a solid filter that has essentially the same 
absorption characteristics as nickel sulfate in water. 
Many compounds including glycerol, ethylene glycol, 
propylene glycol, erythritol, inositol, pentaerythritol, 
and mannitol were tried separately and in various com- 
binations. None except sorbitol was satisfactory, owing 
to either crystallization or poor transmission charac- 
teristics. The nickel sulfate-sorbitol complex has sat- 


MIDDLE ULTRAVIOLET NEAR ULTRAVIOLET VISIBLE 





GALLIUM ARC 


MAGNESIUM SPARK 


MERCURY ARC 


Figure 14. Density and transmission curves for G-filter alone, components of G-filter, and G-filter plus NAT filter. 
Spectra of sources to show radiation in ultraviolet region. 


millimeters of the complex and 3.5 millimeters of the 
Corex makes the gallium lamp invisible at 50 feet 
to a dark-adapted observer. The filter has good trans- 
mission in the region 0.26-0.35 micron, as seen in 
Figure 14. In the region of the strong gallium lines 
(0.28-0.30 micron), its overall transmission is remark- 
ably high, from 65 to 74 per cent. 

The variation in transmission at 0.28-0.30 micron 
is due to the Corex 9863 filters. Different batches of 
these seem to differ considerably. 

While the filter transmission from 0.31-0.35 micron 
would be objectionable for many ultraviolet sources 
because of the resulting fluorescence of the retina of 
the eye, it is of little consequence here because the 


isfactory transmission in the desired region only if it 
is prepared under vacuum at a temperature of 60 C 
or less. Details of the procedure for making the G- 
filter are given in the contractor’s report.^ 

The finished Alter complex is a very sticky mass 
that does not pour but flows under pressure. The over- 
all transmission of the Alter, consisting of 4-milli- 
meter Corex 9863, 3.5-4.0-millimeter nickel-sorbitol 
complex, 3.5-4.0-millimeter quartz, is 69-74 per cent 
at 0.28-0.29 micron. Hence, since the Corex by itself 
has about 80 per cent transmission here, the complex 
absorbs very little in this region. The Alter can stand 
the low temperature of solid carbon dioxide and can 
be heated to 70 C for several hours without any per- 


annwnwnnwnii 


108 


ULTRAVIOLET SOURCES AND FILTERS 


ceptible change, but it will not stand long heating at 
higher temperatures. It stops practically all the radia- 
tion from the gallium lamp except that in the region 
0.25-0.30 microns. It satisfactorily eliminates all the 
visible except for a small amount in the extreme red 
and violet regions (see Figure 14). 

6.4.4 Filter for the Magnesium Spark 

This filter consists of the same “sandwich” used 
with the gallium lamp plus a film of polyvinyl butral 
or cellulose acetate containing a compound that will 
transmit in the region 0.28-0.30 micron and absorb 
from 0.305 to 0.35 micron. 

Many materials were investigated for making the 
films. These include polyvinyl chloride, methyl metha- 
crylate, cellophane, polyvinyl hutral, and cellulose 
acetate. Only the last three had suitable transmission 
in the desired region. Of these the polyvinyl butral 
(Butvar) and cellulose acetate (Hercules Powder 
Company special) had superior transmission to the 
cellophane (Dupont unlacquered), as shown in Fig- 
ure 15. Polyvinyl butral has slightly better trans- 
mission than cellulose acetate but the difference is 
not critical. However, films of cellulose acetate are 
more easily made with good optical properties and are 
also removed more readily from the glass plates. 

The compounds investigated for cutting out the re- 
gion 0.305-0.35 micron include anthranilic acid 
(ONHg), p-dimethylaminobenzaldehyde (PDAB), 
picric acid, auramine 0, p-dimethylaminoacetophenone 
(PDAA), 5-nitro-2-aminotoluene (NAT), MichlePs 
ketone, p-nitroaniline (PNA), p-nitrodimethylaniline 
(PNDA), diphenyloctatetraene, a, a'-dichlorocam- 
phor, methyl anthranilate, and methyl-m-amino ben- 
zoate. The absorption curves for some of these com- 
pounds are given in Figure 14. The most suitable 
compound is 5-nitro-2-aminotoluene, referred to as 
NAT in this chapter. Anthranilic acid has fairly good 
absorption characteristics, but it fluoresces and is 
decomposed by ultraviolet light. p-Dimethylaminoben- 
zaldehyde also has good absorption in the desired re- 
gion, but it has a tendency to escape from the film 
and is also decomposed by radiation. 

6.4.5 Photomultiplier -Tube Receiver Filter 

This filter consists of the nickel sulfate-sorbital 
sandwich with a 6-millimeter thickness of Corex plus 
a fairly concentrated film of NAT in cellulose acetate 
The problem here is to eliminate the daylight radia- 
tion in the visible and near ultraviolet and still to 
have transmission of radiation from 0.30 micron down. 



MICRONS 





Figure 15. Density and transmission curves for 
compounds and film materials. 




FILTERS FOR ULTRAVIOLET SOURCES AND RECEIVERS 


109 


preferably as low as 0.25 inieroii. However, since the 
cellulose acetate or polyvinyl biitral absorbs appreci- 
ably at 0.23 micron and the NAT compound absorbs 
fairly strongly from 0.26 micron down, the radiation 
effectively used is from 0.265 to 0.30 micron. The ab- 
sorption curve of the filter that is most suitable in 
this case is given in F'igure 15. This particular filter 
had a transmission of about 5 per cent at 0.28 micron. 
Its preparation is described in the contractor's report.^ 

A Better Filter 

Last-minute tests with a combination of PDAH 
and PDAB compounds in a Butvar film proved to be 
considerably more effective as a phototube receiver 
filter than the NAT filter described above. It is to 
be noted in Figure 14 that the G -f NAT filter reaches 
maximum transmission of about 10 per cent while 
that of G DDAA -|- DDAB goes up to nearly three 
times this with an equally sharp cutoff of the higher 
wavelengths. 

^ Goggle -Filter for a Mercury Arc 

During the past two years, high-pressure mercury 
arcs have been used to illuminate the fluorescent 
clothing worn by Landing Signal Officers [LSO] on 


night-carrier landings. Each lamp is located on the 
deck a few feet from the LSO. A ^‘black^^ filter passes 
red and violet light and UV down to 0.36 micron; 
the lamps appear purple and the eyes of the LSO 
fluoresce. NDRC was requested to eliminate the visible 
light and to protect the eyes from fluorescing. 

First, a Corning 9840 filter was placed in front of 
each mercury lamp. This green filter cuts out the red 
light, but passes UV. To protect the eyes of the LSO, 
Navy Polaroid clear- vision plastic goggle lenses were 
impregnated with p-nitroaniline. After thorough 
cleaning, each lens was moistened with alcohol and 
dipped for 2 minutes in a 0.5 per cent solution of p- 
nitroaniline in 85 per cent alcohol and 15 per cent 
acetone. After drying for 20 or 30 minutes, each 
treated lens was ready for use. 

The absorption curves of these goggle-filters are 
given in Figure 15, lower right. Compared with gog- 
gles commercially manufactured later to do this same 
job, the NDRC filters have a more complete cutoff of 
the violet and, at the same time, fluoresce by a lesser 
amount, thus giving a more transparent goggle and a 
clearer vision. The NDRC filters were successfully 
used on trial runs, and subsequently a number of the 
Corning filters and dyed goggles were furnished LSO’s 
on several carriers. 


Chapter 7 

AUTOCOLLIMATORS 

By Mary Banning, 


7.1 INTRODUCTION 

A n autocollimatok is a device that returns in- 
^ cident radiation from a distant source back to 
this source in a narrow cone. This enables an observer 
near the source to see the return beam, while an ob- 
server located off the line between the source and auto- 
collimator can see nothing. The more perfect the op- 
tical system of the autocollimator, the more accurately 
this is true. Two types of these devices have been 
developed under Section 16.5 of NDRC for military 
applications where high security is required. 

The Kellner- Schmidt [K-S] system makes use of a 
phosphor to convert incident ultraviolet or infrared 
into a return beam of visible light. This type had 
been proposed and partially developed for peacetime 
applications prior to the formation of NDRC. Specific 
development of ultraviolet conversion autocollimators 
for military use started in the spring of 1941 at the 
University of Rochester and continued under Con- 
tracts OEMsr-69, OEMsr-427, and OEMsr-725. The 
Eastman Kodak Company, OEMsr-994, undertook 
the manufacture of a number of small glass units ; the 
Rochester Button Company, OEMsr-932, developed 
plastic units ; and the Bausch and Lomb Optical Com- 
pany, OEMsr-495, studied methods of molding and 
supplied molds for both glass and plastic units. In- 
frared conversion autocollimators, or metaflectors, 
were developed at the University of Rochester under 
Contract OEMsr-1000. 

A second type of autocollimator, the triple mirror, 
uses either visible or infrared radiation but is not of 
the conversion type. This device has been known for 
a long time and was even used in World War I. Quan- 
tity-production methods for making a precision glass 
form were developed at the Mount Wilson Observatory 
under Contract PEMsr-698. Various applications of 
these were studied and developed at the University 
of Rochester, OEMsr-1219. 

7 2 KELLNER-SCHMIDT AUTOCOLLIMATORS 

^ 2 ^ Ultraviolet Autocollimators 
The principles and advantages of the Kellner- 
Schmidt system have already been described in Chap- 

* Institute of Optics, University of Rochester. 


ter 3, as used in metascopes. When used as an auto- 
collimator, not as a telescope, the system is a great 
deal simpler since no viewing system need be used. 
Invisible radiation entering through the corrector 
plate comes to a focus on the phosphor which then 
emits visible light which is returned to the source 
over the identical path of the incident radiation. 
Ultraviolet-conversion autocollimators have been ap- 
plied to problems of road-marking for traffic operat- 
ing under blackout conditions, to problems of night 
identification, and to the night landing of aircraft 
when the airfield is completely blacked out. Limited 
procurement of two sizes of these ultraviolet autocol- 
limators has been made by the Navy Bureau of Aero- 
nautics. However, the devices have not gone into 
general use because of changes in operational needs 
during the progress of the war. 

Since the rigid requirements of very high resolving 
power necessary for metascopes did not need to be 
met in autocollimators, an //0.52 system was used. 
In the first instruments, shown in Figures lA and 
IB, the phosphor was coated on the convex surface 
of a metal mount which was suspended in the focal 
position of the autocollimator by a rigid spider. These 
instruments have a clear aperture of 4^/^ inches ; later 
models were made on the same principle with 2^- and 
l^A-inch aperture. Solid K-S autocollimators, made in 
the same way as the solid metascopes, have been de- 
veloped and produced on a larger scale in both glass 
and plastic. Considerable development work was car- 
ried on for the mass production of such units.^^*^®’^^ 
Several thousand plastic units and a few hundred 
glass units were supplied to the Navy for test work. 

Ranges obtained with the various kinds of auto- 
collimators vary with the observer and the atmospheric 
conditions. An average production instrument gives 
only about 40 per cent of the range of a similar pre- 
cision model. If atmospheric attenuation of the radia- 
tion is negligible, the range follows an inverse fourth 
power law, so that to double any range, 16 times as 
many autocollimators must be used, a source 16 
times as bright, or a K-S system of twice the aperture. 
A General Electric high-pressure mercury arc was 
developed for this purpose which was 1 inch long, used 


110 




KELLNER-SCHMIDT AUTOCOLLIMATORS 


111 



Figure 1. 43^-inch ultraviolet autocollimator. 

A. Upper — assembled. B. Lower — disassembled. 

400 watts, and operated at 30 atmospheres pressure. 
It was mounted in a standard 7-inch automobile sealed- 
beam reflector. It is described more fully in Chapter 
5. Using the 3650 A radiation and a 4y2-inch autocol- 
limator equipped with a URN-l phosphor described 


below, an average range for fovcal vision of about 1.2 
miles can be attained. 

Phosphors 

As with the metascope phosphors (Chapter 4), the 
most important requirement in an ultraviolet K-S 
autocollimator is that of high efficiency; i.e., it must 
transform as much as possible of the ultraviolet energy 
into visible light, and this light should be of the wave- 
length to which the eye is most sensitive. Also, the 
phosphor should come up to full brilliance as soon as 
the* ultraviolet light falls upon it, with as little delay 
before full emission as possible. Fortunately, a short 
delay usually means an afterglow of short duration. 
Signals may then be picked up immediately, with no 
confusion caused by a lingering glow in a region which 
has been previously illuminated. Another important 
phosphor requirement is that it endure long exposure 
to sunlight Avith impunity. 

Since at low levels of illumination, the sensitivity 
of the eye has a maximum at about 5,100 A, the best 
visual efficiency for a given energy efficiency in a 
phosphor is accomplished when the emitted light has 
an overall greenish blue color. If other colors are de- 
sired for identification purposes, it is necessary to 
sacrifice some visual efficiency. Of course, an individ- 
ual phosphor is usually more sensitive to some ultra- 
violet wavelengths than to others and this, as well 
as the difficulty in making the phosphor and its avail- 
ability, must be taken into account for practical 
applications. 

One of the most useful phosphors for the present 
purpose is a green-emitting cadmium platino-cyanide. 
It was first described many years ago, but its proper- 
ties were fully recognized by personnel of the Physical 
Optics Division of the Naval Research Laboratory. It 
has been further refined at Rochester under Contract 
OEMsr-81. It is most sensitive to ultraviolet radia- 
tion at 3,650 A, but in the later form (URN-1) is 
satisfactory for use around 3,000 A as well as below. 
Potassium uranyl sulfate, also emitting in the green, 
is only about half as efficient as the URN-1 at 3,650 A, 
but it increases in relative efficiency at shorter wave- 
lengths. It is very soluble in water, however, and must 
be protected from moisture. Another phosphor, devel- 
oped by the Continental Lithograph Corporation, 
Cleveland (P-63), has a yellow emission and is used 
when distinction from the green phosphors is necessary. 
Filters 

Contrary to general belief, not all the ultraviolet 
spectrum is invisible to the unaided eye, and advan- 


112 


AUTOCOLLIMATORS 


tage is taken of this fact in the ultraviolet autocolli- 
mators. From 3,200 to 3,800 A there is surprisingly 
little change in eye sensitivity, with the color sensa- 
tion produced in this range an unsaturated blue-violet. 
By utilizing light filters which transmit only the 
strong lines of the mercury spectrum in the neighbor- 
hood of 3,650 A, it is possible to insure enough visi- 
bility of an approaching source for friendly observers 
to detect it, while an enemy observer not in the direct 
beam can see nothing. 

In general, the filter giving the best transmission 
in the 3,650 A region is a combination of Nos. 586 
and 587, or 586 and 984 Corning glasses. For best 
transmission in the 3,130 A region, it is necessary to 
use a liquid filter of nickel chloride solution combined 
with Corning No. 9863. Even with this last combina- 
tion, the lamp source is still visible to the unaided 
eye, and if still shorter wavelengths are used, the ab- 
sorption by the oxygen in the air begins to seriously 
cut down the range. 

Night Landing op Aircraft^’^’® 

One of the first problems at the beginning of the 
war was that of landing aircraft at night with high 
security. To meet the necessary specifications of se- 
crecy, light weight, moderate power requirements, and 
ranges of at least a mile, it was proposed that an ultra- 
violet autocollimating system be used, with the plane 
carrying the source and the autocollimators lining the 
runway. In such a plan, the security of the ultraviolet 
autocollimator system is quite high; the light beam 
is returned in such a narrow cone that it is visible 
only from the cockpit of the plane carrying the source 
and not from any enemy plane no matter how close it 
may be. At the same time, the visibility of the source 
is very low or negligible except to observers on or near 
the landing strip and within the beam of the source. 
Moreover, the pilot is not encumbered by any special 
equipment and makes his approach and landing as 
though the runway were marked out with visible 
light sources. 

Throughout 1941 and 1942, many field tests were 
made of the range of various autocollimators with sev- 
eral ultraviolet light sources and filter combinations 
and with both moving and stationary sources. As a 
result of these tests, the equipment was prepared for 
demonstration to the Armed Forces. The first actual 
landings were made in Rochester in June 1942, and 
soon thereafter demonstrated to both Army and Navy 
personnel. Several glide-path indicators were tried and 
discarded, either because the range was too small or 


because the system required manual operation in the 
field. 

In August 1942, an official demonstration was held 
for representatives of the Army and Navy at Wright; 
Field. The 4^-inch autocollimators were set up 400 
feet apart in two rows, marking a runway 400 feet 
wide; a group of three on each side marked the be- 
ginning of the runway, followed by 9 single ones on 
each side, giving a total length of 3,600 feet. Two 
ultraviolet lamps were mounted in front of the plane. 
Approaches were successfully made from a direction 
of 45 degrees off the center of the runway, with the 
markers becoming visible at a range slightly over 
1 mile. The only objections to the system were the 
difficulty in finding the field from a distance and 
the trouble necessary to install the equipment. It 
was agreed upon by all present that the equip- 
ment demonstrated had performed in a perfectly 
satisfactory manner, and had done all that was 
claimed for it. 

A further test demonstration was held at the Naval 
Air Station at Norfolk in a DC-5 and a fighter F4U4, 
to simulate carrier landings. Here again, the only 
difficulty encountered was the failure to see the auto- 
collimators from the sharp angle of approach necessary 
under these conditions. A final test at the Philadelphia 
Navy Yard, in the spring of 1943, proved that when 
proper alignment of the lamps and autocollimators 
was accomplished, the field of use could be greatly 
extended. Small autocollimators placed on the wings 
of the approaching plane were observed by a signal 
officer on the ground, using an ultraviolet source. 
Ranges with these small autocollimators were unsatis- 
factory, however. 

Night-landing tests with autocollimators were dis- 
continued at this time due to the relatively greater 
efficiency of the triple-mirror landing system described 
later. 

Further Ultraviolet Systems 

A similar system, also developed at Rochester, used 
ultraviolet autocollimators in identifying surface ves- 
sels from aircraft. It was proposed to facilitate sea 
seai'ch by providing friendly ships with autocollimators 
and searching bombers with mercury searchlights.'^ 

Although a searching plane is able by means of 
radar equipment to find and approach a ship by night, 
the bombardier is not able to differentiate between 
small friendly ships and enemy submarines until the 
plane has approached too close to the ship to bomb it. 
Before a second bombing run can be made, a sub- 


KELLNER-SCHMIDT AUTOCOLLIMATORS 


113 


marine has time to submerge. Maintenance of light 
sources or moving parts of instruments on the friendly 
ships was not desired. A range of approximately 2,000 
feet was considered essential, and also the bombardier 
should be able to identify the ship without using any 
optical aid, as the limiting time between identification 
and possible bombing is only a few seconds. 

In the fall of 1942, sea search tests were conducted 
in Chesapeake Bay and at the Norfolk Air Station. 
Sixteen 41^-inch autocollimators were mounted on a 
190-foot trawler, and ranges of 5,000 feet were ob- 
tained consistently. 

A third use for ultraviolet autocollimators was for 
the landing of small boats on hostile islands at night.^ 
Tests conducted at Solomon’s Island, Virginia, showed 
that small solid autocollimators mounted on the masts 
of landing boats enabled one boat to follow another 
with ease. On the flag deck of the mother ship carrying 
the small boats, two 41^-inch autocollimators were 
placed, oriented in such a way that they could be 
viewed very readily from the shore side of the ship; 
an additional unit was hung on a board over the ship’s 
side near the stern, and two more placed on shore. A 
small landing boat was equipped with a 400-watt mer- 
cury-arc lamp filtered to transmit 3,100 A and shorter. 
Ranges of 1 mile were easily obtained, both going to 
shore and returning. 

7.2.2 Infrared Autocollimators, 

or Metaflectors^® 

Work on an infrared autocollimating system, par- 
alleling the ultraviolet, started in 1943. These instru- 
ments have been called metaflectors to distinguish 
them from the ultraviolet autocollimators and from 
the infrared viewing systems, or metascopes. They 
operate basically in the same manner as the ultraviolet 
autocollimators. A high aperture K-S system forms 
an infrared image upon a previously excited phosphor 
sensitive to infrared. Upon this stimulation, visible 
light is emitted which returns through the optical 
system along the path of the incident infrared. Thus 
a distant observer sees the autocollimator emitting 
visible light when he illuminates it with an infrared 
source. The emission of visible light is only in the 
direction of the source wherever that source may be 
in the whole field of the autocollimator. Since it is 
unnecessary for the observer to use any type of view- 
ing device, this system has certain advantages in ap- 
plications where it is important that the observer’s 
view be entirely unobstructed. 


Necessity foe Chaeging 

An important difference between the infrared and 
ultraviolet autocollimators lies in the necessity for 
excitation of the infrared phosphor before use. In con- 
trast to the ultraviolet, which utilizes only the energy 
of the incoming radiation to produce and return vis- 
ible light to the observer, the infrared phosphor must 
be excited by ultraviolet radiation or by alpha particles 
and store this energy of excitation until exposure to 
infrared. This may be accomplished in two ways. The 
phosphor may be excited while in the focal position 
and the high background allowed to die down before 
use, or provision may be made for exciting the phos- 
phor in one part of the instrument and transporting 
it into the working field after excitation. The latter 
method has the advantage that the bright fluorescence 
of the phosphor occurs in a completely shielded part 
of the device, while only the phosphorescent afterglow 
occurs during the time the phosphor is in the working 
position. Thus, for applications where only occasional 
use is required, and that for a short time, the first 
method is most suitable; but for any application re- 
quiring continuous use, or any use where the image 
of a distant source might remain focused for a time 
long enough to exhaust the phosphor within that small 
area, the moving phosphor surface is to be preferred. 

FeATHEB WEIGHT UnIT 

The smallest metaflector designed was intended for 
hand-held operation on a signaling wand for the night 
landing of aircraft. It has an aperture of 1.8 inches 
and is externally excited. An ultraviolet source is con- 
tained in the holder into which the wand supporting 
the small units is thrust when not in use. Each unit 
weighs approximately two ounces and, when used with 
a 12-volt, 450-watt aircraft-landing lamp of 100,000 
to 200,000 beam candlepower and filtered with 6 mil- 
limeters of Corning No. 2540 glass, gives a useful 
range of about 1,800 feet. The phosphor used on the 
focal surface in this smallest type, as in succeeding 
instruments, is Standard VII, described in Chapter 
4. The unit is pressure-sealed but, as an added precau- 
tion, a small silica gel chamber is placed in back of the 
mirror to keep the phosphor dry. 

4%-inch Metaflectoe 

A larger metaflector of the size of the Type B meta- 
scope, with a clear aperture of 4% inches, was next 
developed, intended primarily as a test model for still 
larger instruments to be used as runway markers. This 
uses the moving phosphor type of excitation. The 


114 


AUTOCOLLIMATORS 


plios 2 )lior-coated surface is a segiiieiit of a sphere of 
radius equal to the focal length of the K-S system, 
with the chord of the segment approximately twice as 
long as that required by the working field of the in- 
strument. The sj)herical segment is mounted with its 
axis of symmetry, and that of rotation, so tilted that 
the angle made with the optical axis of the aiitocol- 
limator is equal to half the vertical working field, or 
slightly more than 15 degrees. The spherical button is 
rotated at the rate of two turns per minute by a belt 
drive and appropriate gearing from a 2-watt motor. 
Since it is necessary to use electric power for the ultra- 
violet excitation, a motor is used rather than a clock- 
work mechanism. 

A field width of 50 degrees is obtained by exposing 
one third the area of the phosphor segment in the focal 
surface of the autocollimator, while the remaining 
two thirds is enclosed in a small housing containing 
the exciting source. Ultraviolet, instead of alpha-par- 
ticle excitation, is preferred in the metafiector, because 
the amount of radium necessary to completely charge 
the phosphor every 30 seconds would be excessive. Dur- 
ing rotation, the phosphor is charged by a 2-watt mer- 
cury-arc lamp, in combination with a Corning 'No. 
9863 filter. With this instrument, it is possible to ob- 
tain a range of approximately one-half mile when 
using the 12-volt source and filter described above. 

8-inch Metaflector 

After successful demonstration of the 4%-inch in- 
strument, a larger metafiector was made to obtain 
greater range. This has a clear aperture of 8% inches, 
and is excited in the same manner as the smaller 
model. When used with the same source as the feather- 
weight unit, it has a useful range of the order of 1 
nautical mile. As in all devices of this class, the range 
varies with the fourth root of the beam candlepower 
of the source. The return beam of the instrument is 
very narrow, only a few minutes of arc in width, al- 
though its working field is approximately 30 degrees 
in height by 50 degrees in width. 

The 8-inch metafiector has two marked advantages 
over the 5-inch size. First, the exciting source and ro- 
tating motor with its gear mechanism are no larger 
and no more complicated than for the smaller type. 
Second, the visible light returned by the metafiectors 
varies as the fourth power of the diameter of the en- 
trance aperture, if the aberrations of the optical sys- 
tem are smaller than the beam spread due to scatter 
of light within the phosphor. Thus, doubling the linear 
aperture of the autocollimator is the equivalent of 


increasing the beam candle}3ower of the illuminating 
infrared source by 16-fold, and should double the 
range. Actual field tests confirm this relation within 
the unavoidable errors of field measurement. 

At present, one instrument has been assembled, 
photographs of which are shown in Figures 2A and 
2B. Complete with motor and a sheet-aluminum case, 
this instrument weighs 141/2 pounds. In a completely 
waterproof form for shipboard use, a heavy cast shell 
of aluminum houses the assembly. At the close of the 
war, the Navy was still desirous of obtaining 20 of 
the 8-inch metafiectors for experimental tests. 

7 3 TRIPLE MIRRORS 

^ ^ ^ General Properties 

If three plane mirrors are set mutually perpendic- 
ular to each other, the system possesses the property 
of returning a beam of light back to the source without 
requiring any particular orientation of the mirrors 
to the source. Such devices have been in use for many 
years, including a form in which the three mirror re- 
flections occur internally in a block of glass or other 
transparent material. The latter form permits high 
accuracy in positioning the reflecting faces, resulting 
in high efficiency for the device and a long range of 
operation. These solid forms will be referred to as 
triple mirrors, or triples. Quantity-production meth- 
ods for this precision glass form were developed at the 
Mount Wilson Observatory (OEMsr-698), while vari- 
ous applications for military use were worked out at 
the University of Rochester (OEMsr-1219). Several 
of the applications to military and naval problems 
have been very successful and have resulted in sub- 
stantial service production of triple mirrors and ac- 
cessories. 

A triple mirror is an excellent means of identifica- 
tion and signaling. The observer simply holds a light 
source next to his eye and sees an answering beam 
returned to him by the distant mirror. If the back of 
the triple is silvered, the field is increased from, ap- 
proximately 40 to more than 90 degrees, although in 
this case the intensity of the reflected light is reduced 
by three metallic reflections instead of three substan- 
tially perfect total reflections. However, an aluminum 
coating greatly reduces the polarizing effects found 
with clear glass and the resulting image is improved. 
An ideal triple will spread the return beam over about 
4 seconds of arc ; production triples are now of suffi- 
cient quality to concentrate the light inside a beam of 
approximately 8 seconds. 




TRIPLE MIRRORS 


115 




Figure 2, Eight-inch metaflector. A. Upper — front 
view. B. Lower — side view. 

When observed from the position of the illuminat- 
ing source, the ideal triple acts as a diaphragm through 
which the light appears to come from a point at twice 
the distance from the source to the mirror. Thus, if 
the observer is standing at a point source and can 
barely see a triple at a distance x, the source can just 
be seen at a distance 2x. This ratio of 2 to 1 in range 
between the visibility of the source and that of the 
detector is very desirable, and better than most other 
systems using visible light. However, variable refrac- 
tion in the atmosphere seldom allows this minimum 
ratio to hold for long distances. 

If the triple is of high optical quality, an observer 
can see this virtual source as long as his eye is at 
the real source. The freedom of movement of his eye 
is confined to an area twice the size of the mirror if 
a point source is used. With a large source, the area 


is twice the size of the mirror plus the size of the 
source. As the observer and source move away from the 
mirror along the normal to the entrance face, the in- 
tensity of the reflected image diminishes according 
to the inverse square law applied to twice the dis- 
tance from source to triple. Vignetting occurs when 
off axis. There is a distance away from the triple, 
however, where diffraction begins to ‘^spilh^ the light 
over the edges of the geometrical return beam, and 
then the light is no longer confined to an area twice 
the aperture of the triple and the apparent brightness 
no longer obeys the inverse square law. For a 2-inch 
triple of very high quality, in the absence of atmos- 
pheric disturbances, this spilling occurs at about 1 
mile; increasing the size of the mirror increases this 
limiting range. 

In order to get the best possible performance from 
a triple mirror, the wave surface should not be dis- 
torted by more than a quarter wavelength by reflec- 
tion at each surface. The reflecting planes must also 
be mutually perpendicular to within a few seconds of 
arc. If one of these angles departs from 90 degrees, 
there is a doubling in the return beam. An angular 
error in the prism is multiplied by about 5.3 in the 
return beam. 

Owing to the small weight and permanence of ad- 
justment of a triple, it can be used in many cases 
where other equipment, although perhaps better from 
a security point of view, is banned because of clumsi- 
ness or weight. Many such applications have already 
arisen. Figure 3 shows the triples used in the demon- 
strations described below. At least 6,000 of the size A 
were ordered from Mount Wilson Observatory by the 
Navy Bureau of Aeronautics, and an authorization for 
60,000 of the size A' (size A edged round) was given 
just before the close of World War IT. 


Figure 3. Triple mirrors. Left to right: size A (2 
inch), size B (3 inch), size A' (size A edged round). 


116 


AUTOCOLLIMATORS 


7.3.2 Production of Triple Mirrors^^ 

The manufacture of triple mirrors on a production 
scale was undertaken at the Mount Wilson Observa- 
tory in 1942. Since the basic aim was to develop a sim- 
ple method of production and not necessarily a new 
one, this subject was treated as an engineering rather 
than a research problem. Particular attention was 
given to the various phases of manufacture in order 
to eliminate as far as possible any laborious hand- 
correcting operations. One part of the process, that 
of forming the rough shape by diamond milling, was 
strictly a machining operation adapted to optical work. 
Here the physical shape of the triple was utilized; it 
was made the corner of a cube and its edges milled 
parallel to those of the cube. It was then removed from 
the cube and processed by normal optical shop tech- 
niques. 

A model production line was set up so that the 
final process could be tried out on a limited produc- 
tion scale and appropriate time studies could be made. 
By the end of January 1943, nine people were work- 
ing on the contract, four of them full time. One of 
the primary purposes of the project was to train peo- 
ple who were totally unfamiliar with optical work 
and to record their progress, which was surprisingly 
rapid. 

Equipment 

The following equipment was installed. 

1. Two Blanchard No. 11 grinding machines. 

2. One Norton vertical spindle grinder, for small 
lots. 

3. One Covel grinder, used primarily as a saw. 

4. Three 2-spindle 12-inch grinding and polishing 
machines. 

5. Five 5-spindle 8-inch grinding and polishing 
machines. 

6. Two 6-spindle 8-inch grinding and polishing 
machines. 

The 10-multiple spindle machines were designed 
and constructed in the Mount ‘Wilson instrument 
shop, along with auxiliary equipment, such as colli- 
mators and jigs. 

PliODUCTION 

By the end of March 1943, more than 200 triple 
mirrors had been finished and the production time 
had been reduced from over 20 to about 6 man-hours 
per unit. During 1943, about 1,000 triples of various 
sizes (IV 2 -, 2-, 2 V 2 -J 3-, and 6-inch aperture) were 
completed, and the production time reduced to 3 man- 


hours apiece. In April 1943, the working tolerances 
were reduced from 2 seconds to 1 second of arc with- 
out materially increasing the production time. The 
average glass blank was not good enough to warrant 
such accuracy, however, and internal errors due to the 
quality of the glass gave an error corresponding to an 
angular error of approximately 2 seconds. 

At least one manufacturer (Penn Optical Com- 
pany^®) had attained satisfactory production by the 
middle of 1944, using the general method developed 
at Mount Wilson; this left the Mount Wilson group 
free to devote more time to purely experimental work 
and less to producing triples in quantity. Glass blanks 
of superior quality were produced by the Hayward 
Optical Glass Company of Los Angeles, and the re- 
sulting triples showed extremely small errors. 

Considerable evidence was found that the finished 
triples are subject to a gradual change in shape. These 
changes do not appear in all mirrors and usually re- 
quire several weeks or even months to become appre- 
ciable. Accumulated data indicate that “^creep’^ actu- 
ally occurs, but it cannot be traced to any one manu- 
facturing process. Very often the error due to creep 
amounts to as much as a quarter wavelength. 

An experimental triple mirror, utilizing three tri- 
angular plane mirrors mounted in an adjustable metal 
frame, was completed in order to avoid using an ex- 
cessively thick block of optical glass. This device had 
a clear aperture of 6 inches. Although not particularly 
successful as an optical instrument, it showed that the 
extremely accurate mechanical devices necessary for 
adjustment could be made. 

Process of Manufacture 

The milling procedure is divided into two opera- 
tions : the shaping of the entrance face, and the shap- 
ing of the three side faces. With comparatively small 
lots of glass, say 200 blanks each, about 4^2 minutes 
per triple are required to mill them completely to 
shape. Several minor changes were made in the 
Blanchard grinder to reduce the need of constantly 
attending the machine during milling. The choice of 
a suitable diamond wheel depends largely on the 
area of the individual triples. For small ones, metal- 
bonded diamond wheels are very satisfactory and have 
the advantage of long life; for the larger mirrors, a 
resinoid-bonded diamond wheel was found to be the 
most efficient type, since such a wheel wears just fast 
enough to cut freely. The feed cycle is arranged for 
the 2y2- and 3-inch triples so that the first or rough- 
ing cut is made at 0.060 inch per minute and reduced 


TRIPLE MIRRORS 


117 


to 0.006 inch per minute for the last few thousandths. 
Surfaces produced in this manner are satisfactory for 
further processing, and chipping of the edges is 
eliminated. 

Two types of jigs are necessary. For milling the 
entrance face, a diamond-shaped jig is used, with 
eight sockets shaped to fit the triangular pyramidal 
end of the blank. For milling the reflecting faces, the 
triples are waxed into place on the truncated corner 
of a cube-shaped jig. This jig must be made with great 
accuracy. The waxing is performed with both triples 
and jig preheated to obtain the greatest wax strength 
possible. A special fixture of two accurately aligned 
sockets to fit both jig and glass was made to permit 
accurate location of the blanks on the jigs. 

A production rate of 350 2y2-inch milled triples 
per day is easily maintained. The milling procedure 
holds the angles within 15 seconds of arc and the di- 
mensions within a few thousandths of an inch. 

Many attempts have been made to produce a milled 
surface sufficiently good to allow polishing without the 
necessity of interrrrediate fine grinding. These at- 
tempts have been unsuccessful in general, because of 
random marks probably caused by abrasive particles 
carried in the milling coolant. 

Two of the four triple faces are fine-ground and 
polished in plaster blocks. The most important de- 
partures from the usual finishing technique are the 
use of rather heavy iron rings around the blocks, which 
are left in place while polishing, and a method of 
driving the block with an adjustable spider on the back. 
The entrance face and one of the reflecting sides are 
finished in this manner, with approximately 0.005 inch 
of glass removed from each surface by fine grinding. 

The next step is to mount the triple in a cage for 
polishing and final angle-correction. The face to be 
finished is set into a hole in a glass plate, which forms 
the front of the cage and which is held in place by 
means of plaster; the other faces are left unobstructed 
for testing purposes. During the fine grinding, the 
greater part of the correction for angle is completed. 
As soon as the cage and triple face have been ground 
to a common level, the prism is tested for angle; any 
corrections found necessary are brought about during 
the grinding by clamping weights on a raised metal 
rim of the cage or by inserting the driving pin of the 
machine into one of the several eccentrically placed 
sockets in the top of the cage. It is possible to correct 
angles in this manner to well within 4 seconds of arc. 
During the polishing operation, the angles are cor- 
rected to within the final degree of tolerance. 


The last reflecting face of the triple presents the 
greatest difficulty, for in this case two angles must be 
corrected, and it is necessary to test them both at fre- 
quent intervals. Mounting in cages, fine grinding, 
polishing and angle correction can be accomplished 
in about 1 man-hour per surface. 

The last operation on a given triple is to verify its 
quality by means of an autocollimator. This instru- 
ment consists of a 9-foot-focus telescope with a spe- 
cially constructed compound eyepiece. A boxlike hous- 
ing at the e 3 ^epiece of the telescope contains a very 
thin plane-parallel glass acting as a beam splitter, 
which can be rotated so that the returning image can 
either be visually examined or can be photographed. 
The equivalent focus of the combination is 135 feet, 
enabling the diffraction pattern of the triple to be 
studied. This instrument has proved most useful both 
for final inspection and for providing photographic 
records of the performance of the individual triples. 

7.3.3 Applications to Special Problems® 

Special Night Landing 

In December 1942, the Office of Strategic Services 
presented NDRC with a high-urgency specialized 
problem involving the landing at night of a small 
plane of the Cub type in unfriendly territory. The 
plan was to have a man previously go to the spot on 
foot, prepare a landing strip, and place along it 
markers for the pilot. It was stated that the pilot 
would have no difficulty in locating the general area 
because of certain prominent landmarks. Because of 
the ease of carrying triple mirrors, their simplicity 
and ruggedness, they were an obvious solution to the 
problem. 

Several tests were run at the Rochester Municipal 
Airport, to determine the kind of source necessary and 
the best grouping of the triplets. Four selected triples 
were set 1 foot apart on each side of the beginning of 
the runway, and 5 more on the left side 200 feet apart 
to outline the rest of the strip. A headset consisting of 
two 3-candlepower lamps, covered by red cellophane 
to provide minimum impairment of dark adaptation 
and minimum visibility to distant observers, was worn 
by the pilot. The lamps were restricted to a forward 
beam, reducing the glare on the eye and the back- 
scatter from windshield and cockpit. With this equip- 
ment in a Stinson 105 plane, it was easy to locate and 
hold the runway at 5,000 feet looking through the 
windshield and at 7,000 feet looking through an open 
window. 


118 


AUTOCOLLIMATORS 


These results were communicated to the Office 
of Strategic Services and 2 headsets and 14 triples 
supplied to them. 

Night Laxdixg Guound-Based Planes 

At the request of the Air Force Equipment Board, 
which had been informed of the application of triple 
mirrors to night landing as just described, demon- 
strations were conducted at the Kissimmee Air Base, 
Florida, during April and May 1943. These demon- 
strations used a P-70 airplane. After many tests, it 
was decided to abandon the headsets altogether, be- 
cause of the large amount of scattered light and the 
relatively short range obtainable, and to substitute 
a 4-inch 50-candlepower lamp. This was made as 
small as possible to avoid obstructing the piloPs 
vision, and was mounted directly above his eyes; a 
handle was attached to the lamp so that it could be 
rotated to scan the field. 

For the final demonstration, with both American 
and English observers, the runway was arranged with 
ten triples placed on the ground extending about 600 
feet beyond the near end of the runway, as well as 
additional triples at stations 300 feet apart to line the 
sides of the runway itself. The P-70 plane (landing 
speed of 115 mph) was successfully landed, with the 
pilot reporting that the runway was visible for at 
least 2 miles despite bad weather conditions anti 
looked brighter than when the usual portable small 
incandescent lamps (B-2) were used. 

Night Landing Carriee-Based Planes 

While the ultraviolet autocollimator night-land- 
ing equipment was being demonstrated at the Phila- 
delphia Navy Yard for use with Navy carrier planes 
(Section 7.2.1), it was decided to show the triple 
method also. Approaches were made in a TBD-1 but 
no landing was attempted; the pilot was very enthu- 
siastic about this method and reported the triples 
showed up excellently. As a result of this demonstra- 
tion and the failure of other landing systems to per- 
form satisfactorily, the BuAer requested that the 
triple method of landing at night be further inves- 
tigated. Triple-mirror equipment, including light 
sources, has been furnished and tests have been con- 
ducted by the Navy at Patuxent Biver Naval Air Sta- 
tion. Figure 4 shows a simplified drawing of the land- 
ing scheme, with the path of the plane traced around 
the carrier. 

A bright signal, visible from any direction, must 
be placed on the carrier so that it can be located by a 


returning pilot and inform him whether or not to 
attempt a landing. For these two purposes a cluster 
of 12 size-B triples, arranged in six groups around a 
vertical cylinder, has been designed. A shutter with 
six openings is rotated around the triples so that the 
mirrors appear to blink, to be constantly visible or 
invisible, at the signal officers discretion. The cluster 
is located on top of the island at A. 

If the pilot receives the signal to land, he circles 
the carrier in the manner shown and lines up in the 



Figure 4. Schematic representation of night carrier 
landing by triple mirrors. A, bright cluster of twelve 
triples on island; B, several bright clusters to indicate 
turning point; C, signal officer. 

proper direction by means of several bright clusters 
of triples placed in the neighborhood of B, where he 
begins to make his turn. The signal officer stands at 
C to direct the final approach. For the actual land- 
ing, single triples are observed by the pilot along the 
port side of the deck, with a few scattered ones on the 
starboard side. 

This project was still being developed at the close 
of World War II, and the Navy has indicated a desire to 
carry it to a conclusion. 

Glider Landings 

The problem of landing glider troops in enemy ter- 
ritory at night was the subject of a series of confer- 
ences at Wright Field in August 1943. It was as- 
sumed that paratroopers would land first, clear a 
landing strip and set up markers for glider landings. 
Because paratroopers could easily carry them down 
aiul no auxiliary equipment was necessary, triple mir- 
rors seemed a good solution. 

In a test conducted at Wright Field, the tow plane 
carried an 80,000-beam candlepower spotlight, while 
the glider had a 2,000-beam candlepower spotlight 
mounted at the pilot’s left side for searching pur- 
poses, and a 200-beam candlepower lamp fixed on the 
front of the glider for actual landing. The triples 
weie placed on the ground in units of two, wired to- 
gether with one pointing directly up and the other in 
the conventional manner along the runway. A triple 


TRIPLE MIRRORS 


119 


was also placed on each wing tip of the tow plane, 
facing back. 

The glider was released at 6,800 feet. At an angle 
of 45 degrees, the runway was visible with the 2,000- 
beam candlepower light at any altitude less than this ; 
the usual range later proved to be 1.5 miles at 1,200 
feet. The two triples on the tow plane were easily 
visible to the glider pilot. Both tow plane and glider 
made successful landings. 

Plane-to-Plane Identification 

In July 1944, a request came from the Army Air 
Force for some system of plane-to-plane identifica- 
tion of B-29^s. In bombing attacks over Germany, our 
planes had been disturbed by the infiltration of enemy 
night fighters. It was desired to install some system 
by which the tail gunner of a B-29 could identify a 
plane as friend or foe before that plane had closed in 
to less than 3,000 feet. 

It was proposed that every B-29 carry a triple on 
each wing tip, facing forward. The tail gunner’s com- 
partment would be equipped with a 6-candlepower 
source and a small beam projector, boresighted with 
the gLinsight. He would pick up a trailing plane by 
radar and wait until it approached to a known range 
as determined by an APG-15 unit. The reticle pat- 
tern of the sight would be roughly set (because of 
the great difference between the B-29 wingspread and 
that of any other plane) at the proper separation for 
the apparent wingspread of a B-29 at this range. As 
soon as the range was reached, the gunner would 
momentarily energize the small projector. If no re- 
turn beams were seen, or if they did not fit the re- 
ticle, he would fire. 

In Wright Field tests with Type- A triples, an easy 
range of 1.2 miles was established, and with Type B 
a great deal more light was obtained. In all, six B-29’s 
were equipped with triples and projectors. The system 
has not gone into general use, however, as theater 
operational requirements changed and no further fol- 
low-up was requested. 

Night Tokpedo Bombing Training 

Another triple-mirror application was for training 
pilots for night torpedo bombing. In this training, a 


pilot makes successive radar approaches on a friendly 
ship on a very dark night. It had heretofore been im- 
possible for the pilot or for watchers on the ship to 
tell how well the approach had been made. Two or 
more triple mirrors w^ere mounted on the target at 
widely spaced locations of known separation, and a 
light source and continuously recording camera 
mounted close together on one wing of the bomber. 
These were boresighted with the axis of the plane and 
had sufficiently wide field to more than cover the 
target. Since the pilot could not see the return beam 
from the triples, he was unprejudiced in his flying. 
The film record proved of great use to both the pilot 
and his instructors. 

Night Landing with Triples on Plane 

In many cases when a plane returns to its carrier, 
it is badly damaged, and, although the pilot may think 
his landing gear is down, it sometimes is not. It was 
suggested that triples mounted on the retractible 
wheels, flaps, and tail hook of a carrier plane would 
enable the signal officer to determine whether or not 
the wheels were down; he could then signal the pilot 
whether it would be safe to attempt a landing. 

In this case, triples of size A' are used. The 
signal officer is provided with binoculars upon which 
are mounted one or more small restricted-beam light 
sources. Watertight mountings have been designed 
and water-repellent coatings used on the entrance 
face of the triples. The system can be used either 
with or without the triple system placed on the 
carrier, described above. It was for this use that the 
60,000 triples w^ere desired by the Navy. 

Night Air-Sea Rescue 

The Equipment and Materiel Branch, BuAer, re- 
quested assistance on the application of triples for the 
rescue at night of men down at sea and supported 
either by a life raft or a vest. To enable rescues to be 
made at night as well as by day, triples, which can 
easily be seen by a searching plane, are mounted on 
the heads of the men. Recent extensive air-sea rescue 
tests of this system have been conducted by the Navy 
in the Caribbean, with very gratifying results. 


Chapter 8 

ANTIGLARE DEVICES 

By Mary Banning^ 


S EVERAL ANTIGLARE clevices for special purposes 
were developed under Section 16.5. Graded- 
density goggles^, to be worn by pilots while flying un- 
der conditions of extreme glare, were designed by the 
Bausch and Lomb Optical Company [B&L] (OEMsr- 
989). Three conti’acts were respectively assigned to 
Harvard University (OEMsr-571), Eastman Kodak 
Company (OEMsr-996), and the University of Eoch- 
ester (OEMsr-1219) for the development of special 
instruments as aids in defense against aircraft attack- 
ing from the general direction of the sun. 

8 1 GRADED GOGGLES^ 

In the early spring of 1943, at a meeting of the 
Sub-Committee on Visual Problems of the National 
Research Council Committee on Aviation Medicine, 
Dr. Walter Miles, a member, emphasized the need for 
better sun goggles. Among the samples he exhibited 
was a graded-density goggle he had obtained in Eng- 
land ; it was believed to be of Zeiss origin and a visi- 
tor present at the meeting said that he knew such 
goggles were marketed commercially by Zeiss. They 
apparently consisted of two pieces of glass, one clear 
and one of an unsaturated greenish color, which had 
been ground and polished in a wedge shape so that 
the thickness of the absorbing glass varied linearly 
from the tip to the base of the wedge. Thus, an ob- 
server looking straight ahead would have the entrance 
pupil for his eyes cut practically in half by the tip 
of the neutral wedge. Looking above this point, the 
density would increase in approximately a linear man- 
ner with angular elevation of the eyes. The sample 
exhibited, however, was of much too low density to 
be of any practical value for military work under bad 
glare conditions. 

It was learned that B&L had made a few sam- 
ples after the Zeiss pattern, and one such pair was 
tried under a variety of glare conditions in Cali- 
fornia desert country during April of 1943. Evidently 
the graded goggles had possibilities, but the grada- 
tion in density proved to be uncomfortably rapid near 
the center of the visual fleld, while not rapid enough 

^Institute of Optics, University of Rochester. 


well above the center to provide proper protection in 
flying or driving into a low sun. The gradation in 
density at the center had the unpleasant effect that in 
either rough air or in driving a car over rough roads 
the unavoidable bouncing of the head caused a suffi- 
cient change in light transmission to produce a flicker 
effect which was very disturbing. It was also observed 
on these tests that for use in desert country, flying 
above a dense overcast, or driving over snow some 
protection was needed for glare from below. 

A contract was arranged with B&L (OEMsr-989) 
under Section 16.2, NDRC, to develop graded-density 
goggles. It was contemplated at the time that the 
goggles would be of the Zeiss form, but that perhaps 
glass of stronger absorption would be used and pos- 
sibly other modiflcations made which might improve 
them. 

It was suggested by the section that the solution 
to the flicker effect and the protection from glare from 
below could be accomplished by a single change in de- 
sign. Instead of a wedge, a concave cylindric lens of 
absorbing glass could be fused to a convex cylindric 
lens of clear glass. If the two glasses have the same 
refractive index, and their external surfaces the same 
radius of curvature, such a combination introduces no 
magnification or distortion. By placing the axes of 
the cylinders horizontally, the region of minimum 
density lies across the center of the field with the 
density increasing either above or below. The grada- 
tion in density in this case varies as the square of the 
displacement from the center, and not in a linear 
fashion. This has the advantage of a small variation 
in the center so that head movement and jerks arc 
not troublesome, yet a rapid increase in density takes 
place near the top and bottom. 

A sample of this scheme was immediately con- 
structed by the contractor and tested in the summer 
of 1943. It proved to have a very great advantage 
over the original Zeiss type, and was also better than 
the original B&L sample, but it was rather difficult 
to manufacture. As a result, the contractor undertook 
to produce an equivalent change in optical density 
over the surface of the lens by evaporating a hard film 
of nickel-chromium alloy onto the surface of either 


120 




DETECTION OF AIRCRAFT AGAINST SUN GLARE 


121 


a clear or tinted glass of uniform thickness. Such a 
metallic coating gives an almost perfect ^hientraP^ 
density. The gradation is accomplished by evaporating 
through a rotating mask which alternately exposes and 
protects the glass from the stream of distilling atoms. 
By properly shaping the contour of the mask, any 
desired gradient in optical density can be secured. 

The evaporated-film goggles were just as satisfac- 
tory to use as the cemented or fused glass combina- 
tion, and B&L found them much easier to make. The 
slight disadvantage of a mirror surface on one side 
of the glass could be overcome by forming the goggles 
^rom two pieces of neutral absorbing glass, with the 
evaporated film on the interface between them. Thus, 
any light reflected from the mirror surface would have 
to undergo double passage through the neutral glass 
and would be considerably attenuated. It was believed 
that this might be necessary in close hand-to-hand 
fighting such as in jungle warfare, but for aircraft 
or for most desert operations it was considered that 
the glint of sunlight on the goggles would not be 
sufficiently conspicuous to an enemy to warrant the 
trouble of adding the cover glass. 

In some cases, it is advantageous to have the 
graded-density film only on the top half of the goggles. 
When flying an open cockpit airplane or one with a 
military canopy, there appears to be advantage in a 
graded density below the center line only when look- 
ing out over a bright overcast. In the case of a closed 
automobile or plane, there is a distinct disadvantage in 
having absorption below the center line because there 
is not usually sufficient light within the car or plane 
to render the instrument panel properly visible through 
a graded density. 

Samples of both forms were presented to the Air 
Forces and to a number of other agencies for test and 
trial. Acceptance was quite enthusiastic, with a request 
from the Eighth Air Force in England for immediate 
procurement. This resulted from samples which had 
been sent to the OSRD Liaison Officer in London. 
Substantial numbers of those with graded density 
above the center line only have been procured by the 
Air Force, together with a few hundred of the type 
with graded density in both directions. Although the 
goggles are not a cure-all for every sun-glare condi- 
tion, they appear to cope with a much wider range of 
conditions than any form hitherto proposed. 

Development and manufacture of these goggles were 
classified until early in November 1945 when the 
Air Force authorized B&L to announce that the gog- 
gles had been made for the Armed Forces. 


8 2 DETECTION OF AIRCRAFT 

AGAINST SUN GLARE 

In order to keep in view an object approaching 
from the direction of the sun, it is necessary to be 
able to see the object against the sky background in 
the neighborhood of the sun, and also against the bril- 
liant background of the sun itself, which is between 
10^ and 10® times brighter. It is impossible to select 
any single smoked-glass or light filter which will per- 
mit an airplane silhouette to be seen clearly both 
against the sun’s disk and against the adjacent sky. 
If the filter is made sufficiently dense so that the sun 
is not dazzling, the surrounding sky is completely 
blacked out, whereas if it is made suitable for observ- 
ing the sky, the solar image will be so bright that only 
an airplane very near at hand will be recognizable 
against it. 

This fact is so well known that planes always ap- 
proach an enemy target from the direction of the sun 
whenever possible, in order to avoid visual detection. 
In 1942, a device to counteract the glare of the sun 
by occulting the solar image with a high-density disk 
just large enough to cover the sun’s image was pro- 
posed by the Harvard College Observatory. A similar 
but unsuccessful device was developed by the Eastman 
Kodak Company. Later, a completely different device, 
an Icaroscope, was developed and was in production 
at the close of the war. 

^ 2 ^ Occulting-Disk Method 

If a translucent disk is interposed between the sun 
and the eye of an observer, of a size such that the 
solar disk is just obscured and of a density such that 
the intensity of the solar image is reduced to that of 
the sky, any object between the viewing instrument 
and the sun can be seen equally well against the sun’s 
disk or against the surrounding sky. Such a disk may 
be used with a telescope, and placed to block the solar 
image either in the exact position of the sun’s image 
or beyond the objective lens. Preliminary experiments 
at Harvard Observatory showed that best results were 
obtained with a disk with a central portion of density 
3 occulting the sun and a surrounding region graded 
in density from 3 to zero. The diameter of the com- 
plete disk was three times the diameter of the central 
portion, with the ideal gradation from center section 
to edge lying somewhere between a linear density and 
a linear transmission. Automatic positioning of this 
disk so that it always centered on the sun could be 


122 


ANTIGLARE DEVICES 


most readily controlled by means of a photoelectric 
mechanism. 

Type 1 

The first such instrument, developed under Con- 
tract OEMsr-571 with Harvard University, involved 
a sighting unit, designed for tripod mounting, com- 
prised of a telescope with a movable glass plate sus- 
pended in the focal plane of the primary objective. 
This plate had an occulting area of high density at 
its center, which could be moved to any point within 
the field of view, or to any part of a circumferential 
area surrounding the field of view. Rigidly attached 
to this plate is a photocell holder so positioned that 
when the occulting disk intercepts the sun’s image in 
the viewing system, the center of the photocell is 
directly behind another image of the sun formed on 
a ground -glass plate at the focal plane of an auxiliary 
guiding telescope. Two electric motors, with their 
applied forces mutually perpendicular and controlled 
by the photocell, direct the position of the occulting 
disk. Motors, disk, and photocell are mounted in a 
sheet-metal housing measuring 7i^x9xl2 inches. The 
two telescopes are attached to the 9xl2-inch front 
face, and the eyepiece to the rear face. 

The photocell, made by RCA, has four cathodes, 
each an equal portion of a conical surface. Opposing 
pairs of cathodes are coupled to the input of balanced 
feedback amplifiers each of which in turn regulates 
the firing of a pair of thyratron tubes. Each thyratron 
of one pair has one field of a split-field motor con- 
nected in series with its anode. Unbalanced illumina- 
tion of an opposed pair of photocell cathodes unbal- 
ances the amplifier, causing one thyratron to fire and 
thus controls the direction of rotation of one of the 
motors. In this manner, successive unbalancing of the 
image on the photocell acts as a drive to regain 
balance. 

The instrument was first demonstrated for Navy 
and OSRD personnel at the Naval Observatory in 
Washington on February 2, 1943. With the telescope 
pointing in the direction of the sun at noon, the hori- 
zontal position of the occulting disk was corrected at 
the rate of approximately 45 times per minute, corre- 
sponding to an image displacement of 20 seconds of 
arc per correction. The time required for the occulting 
disk to traverse the entire diameter of the field was 
approximately 0.5 second. 

Conclusions based on this test were that a sen- 
sitivity able to correct for an image displacement of 
30 seconds of arc is sufficient, but that the time re- 


quired for the disk to traverse the diameter of the 
field should not exceed 0.1 second, in order to prevent 
a blinding flash in the eye of the observer. Accuracy 
with this instrument averaged less than 15 minutes of 
arc, whereas an accuracy within 30 seconds of arc 
was desired. 

Type 2 

Another type of device was also proposed, with the 
occulting disk outside the objective of the viewing 
telescope. In this case, the disk must be supported by 
an arm having theoretically a length in feet approxi- 
mately 10 times the diameter of the lens in inches., 
Construction of a unit carrying a 10-foot arm was 
instituted at Harvard University. The unit was later 
redesigned to employ an open sight of zero power 
which would allow a shorter arm. The disk was 
mounted on a turntable and controlled by a photo- 
cell and thyratron system as in Type 1. 

« 2 2 Shutter Method' 

Two methods somewhat similar to the Harvard 
device were developed by the Eastman Kodak Com- 
pany during 1943, under Contract OEMsr-996 ; one of 
these was a photoelectric and mechanical process and 
the other one photographic. 

Photoelecteic Peocess 

The photoelectric process uses an opaque disk in a 
telescope to cover the image of the sun, but differs 
from the Harvard instrument in that a shutter is 
employed to prevent observation unless the disk is in 
the occulting position. Since a barrier-layer photo- 
cell generates about 10 milliamperes of current when 
exposed to the light of the sun’s image, it was hoped 
that the energy obtained from such a cell would be 
capable of operating a small shutter near the exit 
pupil of the eyepiece. A photocell, whose sensitive 
surface is cut away to leave only a wheel-like section 
with sensitive spokes, is rotated rapidly about an 
axis such that the spokes traverse the field lens of the 
telescope. As a spoke intercepts the sun’s image, the 
current rises to a value sufficient to open the eye 
shutter. To avoid objectionable flicker and to allow 
the telescope to be used for reasonably fast tracking, 
the rate of interruption must be about 30 cycles or 
more per second. This development was unsuccessful, 
since it was found to be impossible to produce a shut- 
ter that would operate at the required frequency with 
the available current input. 




DETECTION OF AIRCRAFT AGAINST SUN GLARE 


123 


rilOTOOKAPHlC PkOCESS 

The photographic method consisted of running a 
film through the focal plane of the telescope, allowing 
the sun to produce a dense image of itself which would 
act as the occulting disk. The first method tried was 
that of the piinting-out process in which no develop- 
ment is necessary, but the density obtained in this 


images of the sun and surrounding sky upon a strip 
of 35-mm film coated with the special emulsion. The 
beam passing through is offset by the prism B 
so that the image formed on the film at C will be 
optically identical to the image formed by at D. 
Similar points in images C and D are 0.75 inch apart, 
which is the separation of the standard 35-mm film 




Figure 1. Optical system of photographic antiglare device. 


fashion was only about one tenth of what was needed. 

Another solution was the use of a special emulsion, 
amply sensitive to the intense light present in the 
sun’s image, but low enough in sensitivity to prevent 
exposure from the surrounding sky. The emulsion is 
sufficiently transparent so that its presence in the 
image plane is not objectionable. Such a material may 
be developed by wetting with a special solution; the 
development may be made to proceed very rapidly, 
but whether or not it would be rapid enough was 
problematical. 

Two 14-inch objectives, A^ and A 2 , as shown in 
Figure 1, are mounted with parallel axes to form 


frame. The film is moved intermittently by a standard 
camera pulldown so that a picture exposed at C moves 
next into position D. BetAveen frames C and is a 
wick, E ; this is pushed in contact with the emulsion 
face of the film and wet with a developing solution. The 
solution is intended to develop the latent image formed 
at C during its transport to the frame D. So if in 
any part of the first frame there is formed a latent 
image of the sun’s disk, that image after transport 
and development will be in exact register Avith the 
image formed by A^. 

Lens F acts as a field lens close to the plane of D, 
and G is an eye lens. BetAveen F and G, a sector shut- 


124 


ANTIGLARE DEVICES 


ter synchronized with the film movement covers the 
eye lens until the film comes to rest. 

The trial of this apparatus was disappointing. Devel- 
opment time was too long to produce an image of 
sufficient density at the rate at which the film had to 
move. 

8 2 3 Icaroscope Method^ 

In 1943, a method of using an afterglow phosphor 
was worked out at the University of Eochester, in- 
dependently of NDEC contract, and found to be 


if any brighter than that exposed only to sky light. 

This principle of phosphoreseent saturation is here 
utilized in a telescopic device for viewing the sun and 
surrounding sky. An image is formed on a phosphor 
screen by means of a suitable objective lens, and a 
rotating shutter between lens and screen intermit- 
tently exposes the phosphor to the incident radiation. 
A second rotating shutter, connected to the first, 
exposes the phosphor to view only at such times as 
the shutter in the illumination beam is closed, so that 
the phosphor screen becomes visible only by virtue 



Figure 2. Assembly drawing of Type 1 Icaroscope. 


practical, but no further work was done at the time 
since there was no pressing Service need for such an 
instrument. By the summer of 1944, however, it 
developed that this device, code-named Icaroscope, 
could be of use to the Services, and further refining 
of the instrument was done at the request of Section 
16.5 under existing Contracts OEMsr-81 and OEMsr- 
1219. The image of the sun in the Icaroscope is but 
20- to 50-fold brighter than the image of the surround- 
ing sky. Two sizes of the instrument were designed, 
constructed, and submitted to the Navy Department. 
These permit spotting an airplane under ordinary sky 
conditions at distances well over 25,000 feet. 

The operation of these instruments is based on a 
property observed in many commercially available 
afterglow phosphors; i.e., the apparent fluorescence 
varies approximately as the intensity of the exciting 
radiation, but the phosphorescent afterglow exhibits a 
phenomenon of saturation. Thus, if two areas of the 
same sample are exposed to sky light and direct sun- 
light and then removed to the dark, the phosphores- 
cent afterglow of the area exposed to the sun is little 


of its phosphorescent afterglow. With the phosphor 
used, the image of the clear blue sky is 1 to 2 milli- 
lamberts in brightness while that of the sun is 50. 

Type 1 Icaroscope 

A request for the development of a service type of 
sun telescope was made by Lt. Comdr. E. E. Bur- 
roughs of the Eeadiness Section, COMINCH, in the 
summer of 1944. An assembly drawing of the first 
production prototype Icaroscope, Type 1, is shown in 
Figure 2. A cemented doublet of 4-inch clear aperture 
and 12-inch focal length forms the objective, giving 
an image on the phosphor screen, B. The image of 
a 10-foot diameter circle (airplane cross section) 25,- 
000 feet away is approximately 0.005 inch in diameter. 
On each side of the screen is a rotating shutter E, 
so arranged that the excitation and viewing of the 
phosphor take place at different times. The screen it- 
self is slightly concave toward the objective, to secure 
the best image quality possible. 

Because of the necessity of a very long working 
distance for the eyepiece, which must function as a 


DETECTION OF AIRCRAFT AGAINST SUN GLARE 


125 


simple magnifier, a special aspheric doublet, 1), of 
2-iiicli focal lengths is used. It has a clear aperture of 
30 millimeters, to provide comfortable eye relief. This 
allows room for an erecting prism, C, between the eye- 
piece and phosphor screen, consisting of a special 
roof prism which deviates the axis by 75 rather than 
the usual 90 degrees. This particular deviation has 
been chosen to provide maximum comfort in viewing 
ol)jects anywheie between the horizon and zenith. The 
combination of eyepiece and erector gives an erect 
image of 6-power magnification with a real field of 7 
degrees. 

Fhimination of all scattered light is most important 
in this instrument, because of the tremendous differ- 
ence in brightness of the sun and the sky. Scattering 
by the objective has been cut until 75 per cent of the 
residual scattered light is due to diffraction. Scatter- 
ing by the housing tube and exciting shutter have also 
been cut to a minimum. In order to avoid scattering 
due to exciting light reflected from the back surface 
of the phosphor screen, a glass backing has been 
chosen that transmits the green-yellow emission but 
absorbs the blue, violet, and ultraviolet radiation 
which excites the phosphor. 

Shutter speeds of approximately 2,000 rpm give 
good results. The relative length of time for viewing 
and exciting the phosphor is not critical ; however, the 
shutter cannot be made of equal open and closed 
quadrants. This is because the phosphor screen is 
circular and the illumination sector must be complete- 
ly closed over the whole surface of the phosphor before 
the viewing sector opens, otherwise scattered light 
from a small illuminated area will fog the whole field. 
Thus some shutter space is necessarily wasted. 

The final instrument developed weighs but 9 
pounds complete with motor, and can be used either 
hand-held or in a simple altitude-azimuth swivel 
mount. Operation from a portable storage battery or 
dry batteries is quite feasible since the input require- 
ment of the motor, a Bodine AC 4-10, is only 10 
watts. In the production design of the instrument, 
a silica gel compartment is provided to keep moisture 
away from the phosphor. A contract was placed by 
the Navy Department with the Universal Camera 
Corporation for the production of several hundred of 
these Icaroscopes. Photographs of the production pro- 
totype are shown in Figure 3. 

Type 2 

The design of a smaller, lightweight Icaroscope, 
Type 2, was started in January 1945. It is essentially 



Figure 3. Type 1 Icaroscope, assembled; disassem- 
bled, front view and rear view. 


the same as Type 1, but with an 8-inch focal length, 
2.67-inch aperture, magnification of 4, and a weight 
of 4.3 pounds. In order to save space, two conical ro- 
tating sectors are used in the smaller model. This 
brings the axis of rotation closer to the phosphor 
screen and even more of the sector is wasted than in 
Type 1. For this reason a single aperture is used in 
each section, and dynamic balance is obtained by thin- 
ning a portion of the sector opposite the aperture. 
The motor, a 28-volt DC Eastern Air Devices model, 


126 


ANTIGLARE DEVICES 


lias twice the speed of Type 1, or approximately 4,000 
rpm, which makes up for the single opening. The in- 
strument is easily held in the hand. 

A sample Type 2, which is shown in Figure 4, was 
delivered to the Navy in July 1945. Not shown in 
the photograph is an auxiliary tinder, which also 
deviates the beam by 75 degrees and has a density 
of about 3. This operates at unity magnification, has 



Figure 4. Type 2 Icaroscope, assembled. 


a wider field than the Icaroscope itself, and is used to 
prevent the operator from following his natural im- 
pulse of first looking directly at the sun before using 
the telescope. 

Phosphou Requirements 

In order to perform satisfactorily in an Icaroscope, 
the band of wavelengths emitted by the phosphor 
must be well separated from those wavelengths which 
excite it. This permits the use of a glass backing for 
the screen that will cut off the exciting light while 
letting through the emission. The phosphor must be 
in the form of a fine-grained powder with the best 
possible resolution, for the image of a distant airplane 
cast on the phosphor is very small, and to be well 
resolved this image must fall upon a number of grains. 


Also, the phosphorescent saturation point must be at 
neither too high nor too low an intensity level. If too 
high, the image of the sun will be blinding to the 
eye, and if too low, no contrast will be seen 
between sky and sun. Finally, the time constants of 
phosphorescence must be adjusted; if the decay is too 
slow an image of the sun will remain on the phosphor 
for too long a time and leave a trail as the instrument 
is moved, while if the reaction is too fast the image 
will not have sufficient intensity. 

The phosphor finally developed under Contract 
OEMsr-81 (see Chapter 4) for the Icaroscope consists 
of a zinc sulfide - cadmium sulfide base with a silver 
activator and a sodium iodide flux. All zinc and 
cadmium sulfide phosphors undergo photolytic decom- 
position, which is accelerated by moisture and flux. 
This is prevented by carefully washing the phosphor 
with acetic acid to remove the flux, and by providing 
silica gel to absorb the moisture. 

Formation of Screens 

After the phosphor has been prepared in the form 
of a powder of suitable particle size, it is necessary to 
deposit it on a screen in a uniform layer approxi- 
mately 25 microns thick. This is done by first remov- 
ing the larger grains by elutriation, and then allow- 
ing the fine particles to settle on a glass disk immersed 
in a liquid. Amyl acetate is used for the process, since 
it does not react with the phosphor powder. The elu- 
triation column is adjusted to retain particle sizes 
from about 3 to 8 microns, or about half of the orig- 
inal material. This fine powder is then dispersed in 
ethyl acetate and allowed to settle out on a glass disk. 
After the proper thickness has been deposited, the 
disk is very carefully raised through the liquid. It has 
been found that there is an optimum thickness of the 
phosphor that gives best results, and this is controlled 
by weighing the amount of powder before introduc- 
tion into the settling column. 

Screens formed in this way have approximately a 
17-niicron resolving power, under optimum conditions 
of contrast, when used in the Icaroscope with 12-inch 
focal length lens. 


Chapter 9 


MISCELLANEOUS OPTICAL DEVELOPMENTS 


By Mary Banning'^ 


91 INTRODUCTION 

S EVERAL OPTICAL devices have been developed under 
Section 16.5 of NDRC that are not concerned 
either with infrared or ultraviolet systems. One of 
these, the so-called flash metascope, uses a Type B 
nietascope described in Chapter 3 with a phosphor 
sensitive to visible, not infrared, radiation. Another, 
the Kellner-Sclimidt [K-S] strip projector, uses a 
K-S optical system (also described in Chapter 3) for 
projection, but with a filament in place of the phos- 
phor. The third uses an optical system of projec- 
tion and reception designed specially for the Navy 
CadiUac-2 project and has no relation to other optical 
devices developed under Section 16.5. All three devices 
were being successfully tested at the close of war. An- 
other development was completed in the fall of 1943 
involving the identification of surface vessels from 
aircraft equipped with linked searchlights and anti- 
oscillation mounted binoculars; this succeeded the 
ultraviolet method described in Chapter 7. 

9.2 FLASH METASCOPE^« 

In October 1944, a group of representatives of the 
Army and of Division 16 met to discuss various 
methods of night aerial reconnaissance. Of the several 
methods proposed, that involving the use of flares 
appeared the most promising. At the request of the 
Army, the Institute of Optics (under Contract OEMsr- 
1219) started the development of a system using a 
flash afterglow metascope (flash metascope) for ob- 
servation purposes. 

Flash metascopes were first tried in the spring of 
1941, in connection with the ultraviolet developments 
described in Chapter 7. For the present application, 
the Type B production metascope with 4.75-inch aper- 
ture has been modified with an afterglow phosphor 
selected for the purpose. The reconnaissance scene is 
illuminated with a photoflash lamp or small charge 
of flash powder, which produces an afterglow picture 
on the phosphor surface. This can be viewed and 
studied for a period of 10 to 20 seconds following 

“Institute of Optics, University of Rochester. 


the flash, after which the image dies away to bright- 
ness levels too low to be useful. The metascope has 
been mounted in a specially designed sweep mecha- 
nism to fit the nose of a B-25 or A-26 airplane in such 
a manner as to be used by the bombardier. Tests have 
been made to determine the proper ground illumina- 
tion necessary, using a scale model simulating actual 
ground conditions. 

9.2.1 Metascope Modification 

The use of the metascope for this special applica- 
tion involves a change of the phosphor used on the 
focal surface. Commercial phosphors supplied by the 
U. S. Radium Company were first tried, and later 
replaced by one which has a much better resolving 
power. The best sample consists of a zinc-cadmium 
sulfide hase with copper and manganese as activators ; 
this is quickly excited, has a usable afterglow of 10 
to 20 seconds, and a grain size of 3 to 4 microns. Tests 
with the Type B metascope show that a much brighter 
image can be obtained if the phosphor is pre-excited 
by white or ultraviolet illumination, causing it to show 
an appreciable afterglow. Provision for pre-excitation 
in the metascope is supplied by the light source al- 
ready incorporated in it for infrared use. 

In order to illustrate the effect of using the Type 
B as a flash metascope, and to determine the amount 
and distribution of illumination necessary, a small- 
scale landscape traversed by a simulated convoy was 
set up on the ninth floor of the Rush Ehees Library 
tower, at the University of Rochester. This model was 
shown to iVrmy and OSRD representatives in December 
1944. Observations with the flash equipment were made 
from the thirteenth level with scale models such that 
the equivalent altitude was about 1,200 feet, and flash 
lamps were set off at an equivalent altitude of 400 
feet. The metascope was pointed directly at the scene 
to be viewed. Eyes were closed while the flash was set 
off and then immediately opened to inspect the phos- 
phorescent image seen on the focal surface of the 
metascope. The image showed a useful duration of 
about 20 seconds. 

Flash equipment for local tests was received from 
the Army, and during the winter of 1944-45 several 


la 


m 


127 


128 


MISCELLANEOUS OPTICAL DEVELOPMENTS 




93 STRIP PROJECTORS 


tests were made. How'ever, with the equipment that 
was furnished, insufficient ground illumination was 
obtained. It was calculated from the model that a 
ground illumination of 0.2-foot-candle-second would 
be necessary and that sharp shadows were essential. 


the instrument to appear, a fixation point is provided 
to keep the observer's eye from wandering and to 
maintain the focus at infinity. This fixation point is 
a cross-hair reticle, projected into the headrest from 


^ Sweep Mechanism 

At the end of December 1944, a conference was 
held at the Army Air Forces Board, Orlando, Flor- 
ida, on this project. It was decided there to build a 
sweej) mount for the metascope, to be fitted into the 
nose of a B-29, providing a total sweep from 45 de- 
grees to the vertical, and controlled by a spring and 
air dashpot mechanism to give a constant projected 
velocity at a fixed distance. Such an instrument has 
been constructed, with controls that change the sweep 
acceleration to allow for various combinations of al- 
titude and ground speed of the plane. 

Because of airplane vibration, as well as the size 
of the instrument and other moving parts, the mount- 
ing frame is made from heavy aluminum angles and 
mounted on a plate which in turn can be fastened to 
the bombsight platform. Tapered mounting ways are 
provided to hold the Type B metascope, or, if desired, 
a small camera. 

The design of the headrest comprised a good por- 
tion of the design problem. Since the instrument 
moves through a sizable angle, it would be impractical 
for the observer to follow it during its sweep. There- 
fore, a fixed headrest is used, into which the eyepiece 
of the instrument swings, and the observer keeps 
his head in position throughout the sweep to be ready 
the instant the eyepiece comes into view. Since he 
does not have anything to look at while waiting for 


Figure 1. The flash metascope in its sweep mount 
with the camera below. 


To meet the need for low-altitude aerial photog- 
raphy at night, using strip cameras such as the Sonne 
S-6 and S-7, some form of lightweight high-intensity 
projector is required. Because a much higher beam 
candlepower is possible in a given size of projector 
when a stigmatic image of a line source is used, this 
type was decided upon rather than a concentrated 


the side and upward to the eye by a small right-angle 
prism which snaps quickly and completely out of the 
field the instant the eyepiece comes into position. 

Figure 1 shows the Type B metascope in its sweep 
mount, with a Kodak 35-mm camera below ; the cam- 
era can be used in the same mount and has a special 
back permitting quick developing of the photographs 
taken. 

Figure 2 shows the method of viewing with the 
flash metascope. 


Figure 2. Method of viewing with flash metascope. 



STRIP PROJECTOR 


129 


source image such as a carbon arc with the addition 
of a beam-spreader to the projector mirror. A very 
compact projector, with an outside case diameter of 
about 11 inches, has been developed using a K-S type 
of optical system. 

Initially, the project was undertaken with the ex- 
pectation of using only visible light for photography. 
However, as a result of the high efficiency of the 
projector plus the high-sensitivity infrared films re- 
cently developed by the Eastman Kodak Company, it 
has been possible to use infrared radiation and there- 
by to obtain much greater security. Photographs have 


aperture, reduced only by the transmission and re- 
flection losses in the optical system. Because of this 
high efficiency, an 8-inch aperture K-S projector with- 
out a beam-spreader yields the same beam-candle- 
power which is produced by a carbon-arc system with 
a beam-spreader and an 18-inch parabola. 

A series of filaments for this system was specially 
developed for the purpose by the Lamp Development 
Laboratory of the General Electric Company, under 
Contract OEMsr-423. Finally used was a 28-volt, 
1,800-watt filament of 37-mil tungsten wire with 18 
turns per inch and 2 inches long (see Section 5.3.2). 



been successfully made at altitudes as great as 1,000 
feet, although best results thus far have been ob- 
tained at altitudes of about 500 feet. 

9.3.1 Development of the K-S Projector 

In the spring of 1944, the Chief of the Photogra- 
phic Laboratory at Wright Field informally requested 
the development of a system of strip projection. Pre- 
liminary investigations showed that a beam of light 
approximately 1.5 by 25 degrees was necessary. A 
K-S projection system was decided upon, consisting 
of a spherical mirror corrected for spherical aberra- 
tion by an aspheric corrector plate placed at its center 
of curvature, and a long tungsten coil curved to follow 
the focal surface. Light from the filament is reflected 
by the spherical mirror out through the corrector 
plate. Such a system requires no beam-spreader, and 
the beam candlepower is equal to the brightness of 
the source multiplied by the area of the projector 


Figure 3 shows the optical system of the //0.53 
K-S strip projector. M is a spherical mirror of 8-inch 
radius, C the aspheric corrector plate, F the tungsten 
filament, and S' a shield to prevent direct light from 
the filament f rom leaving the projector. The clear aper- 
ture of the system is 8.375 inches, and the focal length 
is 4.42 inches. The optical system is mounted in a 
lightweight aluminum housing with forced-draft cool- 
ing provided by a miniature 27-volt blower B, built 
into the projector. Including blower, the total weight 
of the finished projector is approximately 11 pounds. 
When the filament is operated at a color temperature 
of 3400 K, the beam candlepower is about 800,000 
over a solid angle approximately 1.5 by 25 degrees. 

Use of K-S Strip Projector 

The final unit used is shown in Figures 4A and 4B. 
This consists of two projectors mounted with an S-7 
Sonne camera, equipped with an //1. 5 lens of 8-inch 


130 


MISCELLANEOUS OPTICAL DEVELOPMENTS 


aperture which covers a 40-clegree held. It was pro- 
posed to use two of these units together, or four pro- 
jectors in all, to make a photographic record of a 70- 
degree strip on the ground. Preliminary tests with 
the two strip projectors were so satisfactory that (as 
mentioned above) it was decided to use infrared illu- 
mination for the hnal demonstration. A cylindrical 
surface was superimposed on the aspheric corrector 



Figure 4. A. Upper — Two strip projectors mounted with 
camera, side view. B. Lower — Strip projectors, bottom view. 


plate, to astigmatize slightly the projected image. 
This was to prevent too accurate focusing of the hla- 
ment on the ground, which causes uneven illumina- 
tion. The two projectors were arranged with a slight 
overlap to cover a held from approximately 5 degrees 
on one side of the vertical to 35 degrees on the other. 
The slit of the camera was opened wide, since the 
sharp projected strip of light on the ground acted as 
the slit itself. This made the most efficient use of the 
light and eliminated critical alignment between the 
camera and projector. 

In July 1945, tests were made in a B-25 airplane 
at Wright Field of the projector system using a new 
him developed by the Eastman Kodak Company cal- 
led Kxx. The two projectors were covered with Wrat- 


ten No. 88 infrared h Iters. Observers on the ground 
being photographed reported that no scattered light at 
all was visible, and that it was impossible to detect 
the projected light except during the 0.1 second that 
they were in the direct beam and then only if they 
happened to look up at the instant the airplane passed 
overhead. In this case, only a faint deep red light is 
seen for this fraction of a second, and extremely high 
security is thus obtained. 

Successful exposures were made up to altitudes of 
1,000 feet. Figure 5 is a sample photograph taken at 
500 feet at an airspeed of 200 mph on Kxx film with 
a slit width of 0.400 inch. The vignetting at the top 
of the picture can be reduced when the unit is tested 
in an A-26, for which it was designed; the bomb bay 
of the B-25 cuts in on the edge of the oblique beam. 
The camera used was a Sonne S-7 with an //1. 5, 
8-inch lens. Unfortunately, it was still set for visible 
focus when the picture was taken, a fact not known 
until after development, and produced an out-of- 
focus infrared image. Much better results are ob- 
tained when the proper focus is used. 

At the close of the war, the Army was desirous of 
obtaining the complete set of four projectors for 
further testing. 

94 CADILLAC-2^^ 

On June 14, 1945, Section 16.5 was asked to assist 
in a top-priority project, Cadillac-2, by BuAer. 

In large fleet operations. Combat Information 
Centers [CIC] are placed aboard major fleet units to 
pick up information on enemy movements by means 
of radar (AEW) equipment, and to maintain a cor- 
related picture of the disposal of all units. For this 
purpose a large screen is used, on which is drawn a 
general geographical map of the area, together with 
temporary positions of friendly and enemy sea and 
aircraft as determined by radar. Watching the changes 
on the screen enables coordinating officers to transmit 
proper directions to individual units. However, low- 
flying attacks are difficult to detect by shipboard 
radar, and to provide protection against these it was 
proposed to place a CIC unit in a specially con- 
structed room in the bomb bay of a B-17 airplane, 
from which all enemy movements could be detected. 

Whereas a ship has a relatively low speed through- 
out such an area, an airplane moves so rapidly that 
the coordinates of its position are continually chang- 
ing with respect to the map. This requires that some 



CADILLAC.2 


131 



Figure 5. Infrared photograph taken with strip projector (see text). 


method be provided of superimposing the changing 
position of the CIC airplane on the screen map. 

A moving grid of polar coordinates, with the B-17 
at the origin, must be projected on a screen which also 
has temporary markings of unit positions and fixed 
markings of geographical points. The maximum size 
allowed for the screen was a circle 4 feet in diameter, 
and the greatest possible projection distance was 81 
inches. It was found that the projector conld he 
placed on the forward bulkhead, and that the screen 
could be slightly tilted so that a line from the projec- 
tor to the center of the screen would be perpendicular 
to the screen and enable a 20-degree half field to be 
used. Optical specifications of a projection system and 
the form of screen to he used were requested by the 
Navy, with the deadline placed at July 1, 1945. 

As planned, the CIC room has a scriber sitting in a 
doorway of the aft bulkhead (No. 5), receiving in- 
formation as to the disposition of enemy and friendly 
aircraft from two radar observers stationed behind 
him outside of the room; he plots this information on 
the screen. The projector is mounted outside the room 
on the starboard side of the forward bulkhead (No. 
1), in such a position that the beam of light passes 


just over the head of one of the four coordinating 
officers. 

^ ^ ^ Screen Specifications 

After several attempts with fluorescent screens and 
nltraviolet illumination, these were discarded as giv- 
ing too faint a response for use and it was decided to 
nse visible projected light. Since this will only show 
up against a diffusing surface, one side of the screen 
must be ground or made otherwise diffusing. This 
prevents edge illumination of the screen, and so it 
was decided to make the screen of two pieces mounted 
close together. As finally recommended, the main 
plotting screen consisted of two layers of Plexiglas, 
both polygonal to facilitate mounting, with a mini- 
mum of 8 sides and a 48-inch diagonal. The front 
surface of the layer facing the projector is ground, 
and the whole illuminated with ultraviolet light. Geo- 
graphical markings can then be written on the ground 
surface with a phosphor pencil, and the moving grid 
can be projected on this surface with visible light. 
In order that the scrilier on the opposite side may not 
l)e confused by ultraviolet-induced fluorescence of his 
eyes, this first layer was made of an ultraviolet ah- 



132 


MISCELLANEOUS OPTICAL DEVELOPMENTS 


sorbing Plexiglas No. 106 with the back surface 
polished. The recommended thickness is preferably 
but possibly Vs inch. 

In close mechanical but not optical contact with 
the first layer is a second of V^-inch clear Plexiglas. 
This is polished on both surfaces and edge illumi- 
nated, with care being taken to shield the first layer 
from the illumination. The scriber writes with a 
grease pencil on this back plate. Cementing the two 
layers together, even with cement of the lowest re- 
fractive index obtainable, is not recommended be- 
cause of the scattering of light from high-index 
points. Stress is placed on the necessity of using as 
thin plates as possible to avoid diffusion of the writing 


photographed the coordinate grid G on a lantern slide 
plate, with lines which when projected on the screen 
are V^-inch wide (magnification, 40 times). Auto- 
matic motion of the slide keeps the center of co- 
ordinates projected on the screen in the geographical 
area corresponding to the position of the CIC plane. 
In order to cover the field of the screen, the slide may 
shift 1.2 inches in the two coordinates in its own 
plane. 

A General Electric 250-watt T-10 projector lamp, 
r, is used as a source ; it operates at 25-28 volts, and 
can be provided with a dimming rheostat adjustable 
to suit the observers. The lamp is sufficiently rugged 
for use in a vibrating plane, and it is strongly recom- 


1 IN, APPROX 
I '—i 



Figure 6. Optical design of projector system; P, projector 

E, Ektar lens; L, auxiliary lens. 

on the back plate due to the ground surface of the 
first. The Eohm and Haas Company, Philadelphia, 
supplies the Plexiglas. 

The scale used is 12 inches to 100 miles, so that 
the total excursion of the screen is plus or minus 
200 miles. 

9.4.2 Projector Specifications 

The requirements for the projector were not as 
difficult to fill as those for the screen. Since the full 
field was set at 40 degrees, the only problem left was 
that of finding a high-speed objective having this field, 
and one quickly available. Figure 6 shows the optical 
design recommended. E is an Eastman Kodak Ektar 
lens. No. 23700, of 50-mm focal length and speed of 
//1. 9, used as the objective. The condensing lenses, 
C, are also obtainable from Eastman Kodak. A heat- 
absorbing glass, A (Corning Dark Aklo), is used in 
the condensing system. 

The Photo Science Laboratory, Navy Department, 



amp; C, Condensing lenses; A, aklo filter; C, moving grid; 

mended that no antishock mounting be used for the 
projector for fear of blurring the image. 

Since the distance from screen to projector may 
vary slightly in different planes, the projectors are 
equipped with a set of auxiliary lenses. The lens, 
L in Figure 6, needed to obtain the exact magnifica- 
tion of 40, is selected from a set of spectacle lenses 
in V4~tfiopter steps up to plus or minus 2 diopters, 
obtainable from B&L. It is placed V4 fi^ch in front of 
the objective. 

The preceding specifications and recommendations 
were reported to the Naval Research Laboratory and 
the Special Devices Division of the Navy, in June 
1945. 

95 SEA SEARCH12.® 

The problem of identification of surface vessels 
from aircraft at night has already been discussed in 
Chapter 7. Both methods described there, the use of 


SEA SEARCH 


133 


Stimsoiiite with visible illiimiiiatioii, and ultraviolet 
autocollimators Avith ultraviolet illumiuatiou, require 
identifying devices on friendly ships. To eliminate the 
need for these, another method was proposed. This 
involves a 6-poAver or 10-power binocular with large 
exit pupil, mounted to avoid oscillation. The mount- 
ing in the airplane must be such that it may be 
directed toAvard the position indicated by radar and 
a searchlight is attached to move Avith the binocular. 
Illumination of the target by the searchlight and 
magification by the binocular allows identification at 
ranges over one mile. 

The use of searchlights mounted on airplanes for 
identification purposes Avas not a neAV idea, and Avas 
regarded Avith skepticism for several reasons. The 
Aveight of the searchlight usually necessitates a mate- 
rial reduction in the bomb load that may be carried; 
the position of the aircraft is immediately apparent 
Avdien the light is turned on; the glare of scattered 
light obscures the target and blinds the pilot; and 
the probability of finding the target Avith the search- 
light beam is relatively small in the time available for 
search. 

Elimination of glare by the use of crossed Polaroids 
over the searchlight and binoculars Avas unsuccessful; 
backscatter Avas greatly reduced, but this was far out- 
Aveighed by the loss of target brightness. By keeping 
the observer as far aAvay from the searchlight as pos- 
sible, and using a light Avith a very narroAV beam, 
difficulties encountered Avith scattered light Avere cut 
to a Avorkable minimum. 

A test Avas held in a PBY-5A airplane over Long 
Island Sound, AAnth a submarine for a target. It was 
found that antioscillation-mounted binoculars were 
better than the unaided eye in locating and viewing 
the submarine, and that a 10x50 binocular Avas def- 
initely better than a 6x40. 

Various searchlights were tested at the University 
of Rochester. A bank of six 450-Avatt, 24-volt aircraft 
landing lights did not give enough illumination for 
use, although the total beam candlepoAver Avas about 
three million. Both carbon-arc and mercury-arc search- 
lights with very narroAV beams were next tried, and 
although both appeared to be usable, the final dem- 
onstration Avas conducted Avith the mercury light be- 
cause of its long narrow beam. It is described below. 

In October 1943, a ground demonstration Avas held 
in Rochester for representatives of BuAer. A small, 
dark gray surface craft was easily identified at a range 
of PA miles under conditions simulating those en- 


countered in sea search. An 18-inch parabolic search- 
light, with a 1,000-watt high-intensity General Elec- 
tric Avater-cooled mercury capsule (Chapter 5) Avas 
used for a source. This gave a beam of approximately 
25 million beam candlepoAver about Vs degree Avide 
and 6 degrees high. The searchlight Avas mounted on 
a rotating table, and connected by synchros to a 6- 
poAver binocular 50 feet away in such a manner that 
the axis of the searchlight and that of the binocular 
remained parallel. It could be turned on and off 
rapidly, and could be rotated at various speeds in 
either direction about a vertical axis. Radar Avas not 
used to detect the small boat, but an infrared source 
aboard the boat Avas observed through a Type A meta- 
scope (Chapter 3), so that the function of the radar 
Avas adequately simulated. 

An observer was able to point the binocular and 
searchlight in the general direction of the boat by 
observation Avith the infrared veiwing system. The 
searchlight A\^as then turned on and the final adjust- 
ment of direction of the binocular-searchlight com- 
bination Avas made. Because of the shape of the beam 
it Avas unnecessary to adjust the altitude of the search- 
light, Avhich effectively reduced the searching prob- 
lem to one dimension, and the probability of finding 
the target Avas greatly increased. The remote control 
for the searchlight, including the shutter and arc, 
was operated by the observer stationed at the bin- 
ocular. An illuminated reticle in the eyepiece of the 
binocular aided in centering the target and also 
showed the exact direction of the searchlight beam. 

No effort Avas made to make the apparatus demon- 
strated usable in airplanes, other than to choose that 
Avhich, Avith straightforAvard engineering, could be so 
adapted. When the equipment Avas turned over to the 
Navy for further development, the folloAving points 
had been established to the satisfaction of the con- 
tractor : 

1. The hinocular-searchlight combination is prac- 
tical for sea search. 

2. Magnification is essential. Ten-power binoculars 
are preferred to 6-power. 

3. The searchlight should have a narroAV sharp- 
edged beam; it should go on and off quickly Avith a 
maximum lag of 4 seconds, and it should be as far 
aAvay from the observer as possible. 

4. The linkage between the radar and the binocu- 
lars and that betAveen the binoculars and searchlight 
must be accurate and fast. 




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GLOSSARY 


Activator. A component of a phosphor, present in small 
quantities only, which may control the emission, excitation, 
or stimulation spectrum. 

Activator, Auxiliary. An activator that controls the stimu- 
lation spectrum. 

Activator, Dominant. An activator that controls the 
emission spectrum. 

Aftergloav (background). Spontaneous emission of a phos- 
phor after excitation ceases. 

Aspheric. Any surface which is neither plane nor spherical. 

Atmospheric Attenuation. The decrease in radiant flux 
produced by absorption and scattering during traversal 
of a given atmospheric path. 

Attenuation Coefficient. The logarithm of the ratio of 
the intensities of a parallel beam of radiation incident on and 
emergent from, respectively, unit length of attenuation path. 

Background. See afterglow. 

Basic IMaterial. The component of the phosphor present in 
largest quantity. 

Beam Candlepower. The candlepower of a source in a given 
direction, when the source is at such a distance that the 
inverse square law applies. 

Blitz. Radium-impregnated gold foil used for exciting 
phosphors. 

Button. The focal surface of a Kellner-Schmidt system coated 
with phosphor. 

CAM. Cloud attenuation meter. 

Contrast. The ratio of the difference in brightness of two 
objects to the brightness of the more brilliant object. 

Corrector Plate. An aspheric surface in a Kellner-Schmidt 
system that corrects for spherical aberration of the mirror. 

Directional Indicating System. A system which indicates 
the orientation of a given line through an object with respect 
to a reference direction in space. 

Dropping. A glass-molding process wherein the glass is heated 
sufficiently to drop into a mold, by gravity or with suction. 

Excitation. The irradiation of a phosphor which causes it 
to emit radiant energy, or, in the case of infrared phosphors, 
to store it for emission at a later time. 

Exhaustion. The decrease in sensitivity of a phosphor due to 
infrared stimulation. 

Extinction. The decrease in sensitivity of a phosphor due to 
both infrared stimulation and quenching. 

Flux. A chemical material which, added to the basic material 


in a phosphor, causes the formation of a definite matrix and 
improves the luminescence. 

GPI. Glider position indicator. 

Greenblock. a refractive material used in the dropping 
process as a mold for corrector plates. 

Kellner-Schmidt. K-S system — a wide-aperture optical 
system with an aspheric corrector plate placed at the center 
of curvature of a spherical mirror. 

Inertia. The delay of maximum emission over stimulation. 

Luminescence. Emission of a phosphor during excitation. 

Metascope. An infrared-viewing device making use of a 
K-S system and an infrared sensitive phosphor at the 
focal surface. 

Photoconductive Cell. A radiation-sensitive detector, the 
conductivity of which increases upon exposure to radiation 
within a restricted wavelength region. 

Positional Indicating System. A system which indicates 
the position of an object with respect to a reference position 
in a two-dimensional space field. 

Quenching. The decrease of sensitivity of a phosphor due to 
some radiation and without the emission of visible light. 

Regeneration. Restoration to a phosphor of sensitivity 
which has been lost by pulverization or shearing stresses. 

Spontaneous Emission. Emission without stimulation or 
quenching, after excitation. 

Stimulability. Any arbitrary measure of sensitivity to stimu- 
lation proportional to the quantum efficiency with respect 
to incident light. 

Stimulation. Release of radiant energy through irradiation 
of an excited phosphor by means of any radiation which is 
not interpreted as being due to excitation. Usually, in this 
report, release of light from an excited phosphor by 
infrared radiation. 

Thalofide Cell. A photoconductive cell in which the radi- 
ation-sensitive surface is prepared from thallous sulfide. 

Threshold Contrast. The smallest value of the contrast 
which may be detected by the eye. 

Threshold Sensitivity. The value in nautical-mile candles 
for which the image in a given metascope just disappears 
extra-foveally ; in phosphors, the reciprocal of the radiant 
flux or radiant energy causing a just perceptible emission 
of a given phosphor under given conditions. 

Time-Lag. The delay of emission after stimulation has ceased. 

Visual Range. The limiting range at which a large black 
object may be seen on the horizon. 



135 









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BIBLIOGRAPHY 


Numbers such as Div. 16-421-M3 indicate that the document listed has been microfilmed and that its title appears in 
the microfilm index printed in a separate volume. For access to the index volume and to the microfilm consult the 
Army or Navy agency listed on the reverse of the half-title page. 


Chapter 2 

INFRARED IMAGE TUBES AND 
ELECTRON TELESCOPES 

1. Infrared Image Tubes and Electron Telescopes, G. A. Mor- 

ton, OSRD 5653, OEMsr-440, Service Projects CE-34, 
NS-172, and others. Final Report 16.5-123, RCA Labora- 
tories, Nov. 30, 1945. Div. 16-421-M3 

la. Ibid., p. 56. 

lb. Ibid., p. 53. 

l c. Ibid., p. 81. 

l d. Ibid., pp. 60, 93-95. 

le. Ibid., pp. 63-68. 

l f. Ibid., pp. 177-182. 

l g. Ibid., p. 159. 

l h. Ibid., pp. 101-103. 

li. Ibid., pp. 154-155. 

2. “Applied Electron Optics,” V. K. Zworykin and G. A. 
Morton, Journal of the Optical Society of America, Vol. 26, 
No. 4, April 1936. 

3. “Electron Optics of an Image Tube,” G. A. Morton and 
E. G. Ramberg, Physics, Vol. 7, December 1936, pp. 181- 
189. 

4. Development of Infrared Telescope, G. A. Morton, OSRD 
744, OEMsr-440, Research Project PDRC-269, RCA 
Manufacturing Co., Inc., June 22, 1942. Div. 16-421-Ml 

5. Night Landing Operations with the Aid of Infrared Lights 

and Viewing Equipment, G. A. Morton, OSRD 1046C, 
OEMsr-440, Research Project PDRC-269, RCA, Novem- 
ber 1942. Div. 16-434. 1-M2 

6. Night Driving by Means of Infrared Telescopes ([Part] I), 
G. A. Morton, OSRD 1909, OEMsr-440, RCA, July 1943. 

Div. 16-431. 1-Ml 

7. Infrared Identification Systems Using Electron Telescopes, 
G. A. Morton, OSRD 1910, OEMsr-440, RCA, July 1943. 

Div. 16-440-Ml 

8. Application of Infrared Radiation in Gun Ranging and 
Vehicle Driving under Cover of Darkness, Field Trials at 
Princeton, N. J., on March 27, 1943, and at Fort Knox, 
Ky., April 13 and I4, 1943, G. E. Meese, OEMsr-423, 
Research Project Pr)RC-403, Interim Report 16.5-47, 
General Electric Co., Aug. 17, 1943. Div. 16-431. 1-M2 

9. Application of Infrared Telescopes to Glider and Airplane 

Operation ([Part] III), G. A. Morton, OSRD 1877, OEMsr- 
440, RCA, July 1943. Div. 16-433-Ml 

10. Reconnaissance and Observation of Extended Objects with 
Infrared Telescopes, G. A. Morton, OSRD 1854, OEMsr- 
440, Progress Report V, RCA, August 1943. 

Div. 16-460-Ml 

11. Headmounts for Infrared Telescope for Aircraft and Vehicle 
Use, OSRD 3310, OEMsr-1075, Progress Report 16.5-68, 
University of Pennsylvania, Feb. 15, 1944, 

Div. 16-421. 11-Ml 

12. Driving Test of Red Headlights, OSRD 3336, OEMsr-1075 ' 

and OEMcmr-209, Progress Report 16.5-69, University 
of Pennsylvania. Div. 16-431.3-Ml 

13. Use of Head-Mounted Infrared Binoculars in the Landing 
of Aircraft, OSRD 4092, OEMsr-1075, Progress Report 
16.5-91, University of Pennsylvania, Aug. 18, 1944. 

Div. 16-433. 1-M6 


14. Night Landing of Aircraft with Infrared Radiation, Dem- 

onstration at Lancaster, Pa., on June 7, 1944, G. E. Meese 
OSRD 4945, OEMsr-423, Interim Report 16.5-103, Gen- 
eral Electric Co., June 1944. Div. 16-433. 1-M5 

15. Nocturnal Shoreline Reconnaissance with Infrared Radia- 
tion, Tests at Cape Henlopen, Delaware, March 16 to 17, 
1945 [and at] Fort Pierce, Florida, April I4 to 20, 1945, 
C. L. Amick and G. E. Meese, OSRD 5376, OEMsr-423, 
Service Projects AC-226, CE-34, and NA-175, Progress 
Report 16.5-113, General Electric Co., Aug. 15, 1945. 

Div. 16-434.2-Ml 

16. Infrared Radiation as an Aid in Nocturnal Airborne Oper- 
ations, Wright Field, Stout Field, and Camp Mackall, N. C., 
Three Chronological Reports Covering Development of the Air- 
borne Beacon, A ug. 10 to 19, 1944, Oct. 20 to 24, 1944, Dec. 5, 

1944, G. E. Meese, OSRD 5650, OEMsr-423, Service 
Projects CE-34 and NA-175, Progress Report 16.5-120, 
General Electric Co., Sept. 5, 1945. Div. 16-433.2-Ml 

17. Trial of Infrared Devices in the Detection of Hidden Japanese 
Defenses, [Tests at] Fort Knox, Kentucky [on] July 9 to 11, 

1945, G. E. Meese, OSRD 5920, OEMsr-423, Service Proj- 

ects CE-34 and NA-175, Progress Report 16.5-121, General 
Electric Co., Sept. 17, 1945. Div. 16-460-M2 

18. Development of a Head-Mounted Infrared Binocular Tele- 
scope for Vehicle Driving and for Landing Airplanes, D. W. 
Bronk, H. K. Hartline, and others, OSRD 5654, OEMsr- 
1075, Service Project CE-34, Final Report 16.5-124, Uni- 
versity of Pennsylvania, Sept. 30, 1945. 

Div. 16-421. 11-M2 

19. Activities under Contract OEMsr-423, G. E. Meese, OSRD 

5651, OEMsr-423, Service Projects AC-226, CE-34, and 
NA-175, Final Report 16.5-127, General Electric Co., 
Sept. 25, 1945. Div. 16-430-Ml 


Chapter 3 

METASCOPES 

1. Development of Kellner-Schmidt Optical Systems (Parts I 
and II), OEMsr-510, Interim Reports 16.5-16, and 16.5- 
17, University of Rochester, June 30, 1943. 

Div. 16-421. 21-Ml 

2. Outline of Metascope Infrared Telescope Development, 

OEMsr-81, and OEMsr-510, Report 16.5-22, University 
of Rochester. Div. 16-421.2-M3 

3. Development of Infrared Telescopes Utilizing Infrared- 

Sensitive Phosphors, OSRD 3129, OEMsr-81, and OEMsr- 
510, Report 16.5-55, University of Rochester, June 15, 

1943. Div. 16-421.2-Ml 

4. Development of Infrared-Sensitive Phosphors [covering 

period from] January 1, 1943, to August 31, 1943, OSRD 
3134, OEMsr-81, Report 16.5-57, University of Rochester, 
Aug. 31, 1943. Div. 16-411. 1-Ml 

5. The Type F Metascope, OSRD 3266, OEMsr-81, and 

OEMsr-510, Report 16.5-65, University of Rochester, 
Jan. 1, 1944. Div. 16-421. 23-Ml 

6. Metascopes, OSRD 3400, OEMsr-1100, Problem DD- 

2510C, Report 16.5-73, Eastman Kodak Co., Sept. 22, 
1943. Div. 16-421. 2-M2 


137 


138 


BIBLIOGRAPHY 


7. Development of Optical Device, Type B, formerly Meta- 

scope, Model B [covering period from] June 8, 194-3 to 
February 1, 1944, OSRD 3408, OEMsr-1100, Problem 
2o.lOC, Report 16.5-76, Eastman Kodak Co., Feb. 1, 
1944. Div. 16-421. 22-Ml 

8. Special Optical Devices, OSRD 5656, OEMsr-r219, Serv- 
ice Projects AC-225, NS-172, and others. Report 16.5-126, 
University of Rochester, Nov. 1, 1945. Div. 16-420-Ml 


Chapter 4 

1. Preparation of Infrared Phosphors, [covering the period 

from] March 1, 1943 to August 31, 1943, OSRD 3140, 
OEMsr-982, Report 16.5-56, Polytechnic Institute of 
Brooklyn, Aug. 31, 1943. Div. 16-41 1.1-M2 

2. Development of Infrared Sensitive Phosphors, [covering the 

period from] January 1, 1943 to August 31, 1943, OSRD 
3134, OEMsr-81, Report 16.5-57, University of Roches- 
ter, Aug. 31, 1943. Div. 16-411. 1-Ml 

3. Preparation of Stimulated Zinc Sulfide Phosphors, Report II, 
Characteristics, G. R. Fonda, OSRD 3371, OEMsr-1155, 
Report 16.5-70, General Electric Co., Dec. 7, 1943. 

Div. 16-411. 11-Ml 

4. Survey of Zinc Sulfide Phosphors Capable of Stimulation, 
G. R. Fonda, OSRD 3947, OEMsr-1155, Progress Re- 
port 16.5-84, General Electric Co., July 7, 1944. 

Div. 16-41 1.11-M2 

5. Stimulation and Phosphorescence of Zinc Sulfide Phos- 
phors, G. R. Fonda, OSRD 4587, OEMsr-1155, Progress 
Report 16.5-99, General Electric Co., Dec. 2, 1944. 

Div. 16-411. 11-M3 

6. Preparation and Characteristics of Zinc Sulfide Phosphors 
Stimulated by Infrared, G. R. Fonda, OSRD 5377, OEMsr- 
1155, Service Project NS-172, Final Report 16.5-11, Gen- 
eral Electric Co., July 20, 1945. Div. 16-41 1.11-M4 

7. Infrared Stimulation of Phosphors, [covering period from] 
August 1, 1943 to June 30, 1945, N. F. Miller, OSRD 5360, 
OEMsr-740, Service Projects CE-11, and NS-172, Final 
Report 16.5-114, New Jersey Zinc Co., June 1945. 

Div. 16-41 1.1-M3 

8. Preparation of Infrared Phosphors, [covering the period 

from] September 1, 1943 to August 31, 1945, Roland Ward, 
OSRD 5643, OEMsr-982, Service Project NS-172, Prog- 
ress Report 16.5-119, Polytechnic Institute of Brooklyn, 
Sept. 1, 1945. Div. 16-411. 1-M4 

9. Phosphors, OSRD 5655, OEMsr-81, Service Projects CE- 
11, NS-172, and NS-282, Final Report 16.5-125, Uni- 
versity of Rochester, Sept. 24, 1945. Div. 16-41 1.1-M5 

10. Infrared Phosphors, OSRD 6057, OEMsr-440, Radio Cor- 
poration of America, Nov. 30, 1945. 


Chapter 5 

1. Infrared Equipment for Night Operation of Tanks, V. J. 

Roper and G. E. Meese, OEMsr-423, Research Project 
PDRC-403, Progress Report 16.5-9, General Electric Co., 
Sept. 4, 1942. Div. 16-431-Ml 

2. A Field Demonstration of Infrared Equipment in the Vicinity 
of Solomon’s Island, Virginia, OEMsr-427, Report 16.5-10, 
University of Rochester, Aug. 25, 1942. Div. 16-434. 1-Ml 

3. Application of Infrared Radiation in the Operation of Rail- 

way Locomotives under Blackout Conditions, G. E. Meese, 
OSRD 2033, OEMsr-423, Research Project PDRC-403 
(Division 12, Report 165-72-825E), Report 11, General 
Electric Co., July 23, 1943. Div. 16-432-Ml 


4. Application of Infrared Equipment to Night Amphibious 

Operations, Part I, Field Demonstration in the Vicinity of 
Fort Storey, Virginia, on the Night of February 7, 1943; 
Part II, Field Demonstration at Camp Edwards, Massa- 
chusetts, on the Night of February 23, 1943, G. E. Meese, 
OSRD 2017, OEMsr-423, Research Project PDRC-403 
(Division 12, Report 165-72-824E), Report 12, General 
Electric Co., Apr. 16, 1943. Div. 16-434. 1-M3 

5. Application of Infrared Radiation in Gun Ranging and 
Vehicle Driving under Cover of Darkness, G. E. Meese, 
OEMsr-423, Research Project PDRC-403, Interim Re- 
port ‘16.5-47, General Electric Co., Aug. 17, 1943. 

Div. 16-431. 1-M2 

6. Blackout Driving, Ultrairiolet System, as Demonstrated at 
Palmerton, Pa., on September 27, 1943, [covering period 
from] July 1 to October 1, 1943 (Joint Report), OSRD 3208, 
OEMsr-423, and OEMsr-740, Report 16.5-61, New Jersey 
Zinc Co. and General Electric Co., December 1943. 

Div. 16-431. 2-Ml 

7. Application of Infrared Radiation in the Operation of Rail- 

way Locomotives under Blackout Conditions, Field Trials 
Conducted on the Evenings of October 2, 3, 4, cind 23, 1943 
at Camp Claiborne, Louisiana, {Camp Claiborne Test), 
G. E. Meese, OSRD 3319, OEMsr-423, Research Project 
PDRC-403, Supplementary Report 16.5-66, General Elec- 
tric Co., Feb. 2, 1944. Div. 16-432-M2 

8 . Application of Infrared Radiation in the Operation of Rail- 
way Locomotives under Blackout Conditions, Demonstration 
to Representatives of the Transportation Corps, U. S. Army 
at the Engineer Board, February 22, 1944, {Engineer Board 
Tests), G. E. Meese, OSRD 3946, OEMsr-423, Research 
Project PDRC-403, Supplementary Report to 16.5-85, 
General Electric Co., June 30, 1944. Div. 16-432-M3 

9. Road Delmeation and Nocturnal Vehicular Driving with 
Ultraviolet Radiation, G. E. Meese, OSRD 3945, OEMsr- 
423, Research Project PDRC-403, Final Report 16.5-86, 
General Electric Co., Apr. 18, 1944. Div. 16-431.2-M2 

10. Night Landing of Aircraft with Infrared Radiation, Dem- 

onstration at Lancaster, Pa., [on] June 7, 1944, G. E. Meese, 
OSRD 4945, OEMsr-423, Interim Report 16.5-103, Gen- 
eral Electric Co., June 1944. Div. 16-433. 1-M5 

11. Nocturnal Shoreline Reconnaissance with Infrared Radia- 
tion, C. L. Amick, and G. E. Meese, OSRD 5376, OEMsr- 
423, Service Projects AC-226, CE-34, and NA-175, Prog- 
ress Report 16.5-113, General Electric Co., Aug. 15, 1945. 

Div. 16-434.2-Ml 

12. Night Landing of Aircraft with NAN Markers, Tests at 
NAAF, Charlestown, R.I., May 12 [to] June I4, 1945, 
C. L. Amick and G. E. Meese, OSRD 5564, OEMsr-423, 
Service Projects CE-34 and NA-175, Final Report 16.5- 
118, General Electric Co., Sept. 7, 194^ Div. 16-433. 1-M7 

13. Infrared Radiation as an Aid in Nocturnal Airborne Oper- 
ations, G. E. Meese, OSRD 5650, OEMsr-423, Service 
Projects CE-34 and NA-175, Progress Report 16.5-120, 
General Electric Co., Sept. 5, 1945. Div. 16-433. 2-Ml 

14. Trial of Infrared Devices in the Detection of Hidden J apanese 
Defenses, [Tests at] Fort Knox, Kentucky [on] July 9, 10, 
and 11, 1945, G. E. Meese, OSRD 5920, OEMsr-423, Serv- 
ice Projects CE-34 and NA-175, Progress Report 16.5- 
121, General Electric Co., Sept. 17, 1945. Div. i6-460-M2 

15. Activities Conducted under Contract OEMsr-423, G. E. 

Meese, OSRD 5651, OEMsr-423, Service Projects AC-226, 
CE-34, and NA-175, Final Report 16.5-127, Engineering 
Division, Lamp Department, General Electric Co., Sept. 
25, 1945. Div. 16-430-Ml 

15a. Ibid., p. 49. 

15b. Ibid., p. 69. 


BIBLIOGRAPHY 


139 


Chapter 6 

1. Visual Thresholds at Low Brightnesses^ [covering period from] 
March 1 to October 6, 1943, L. J. Reimert, OEIVIsr-740, 
Final Report 16.5-79, New Jersey Zinc Co., Oct. 12, 1943. 

Div. 16-470-Ml 

2. Evaluation of Ultraviolet Telescope, [covering report from] 

May 7 to October 27, 1943, L. J. Reimert, OEiMsr-740, New 
Jersey Zinc Co., Oct. 27, 1943. Div. 16-421-M2 

3. Visibility of Ultraviolet Light Sources and Ranges of Fluores- 

cent Retrodirectional Autocollimating Demces, [covering 
period from] March 12 to April 12, 1943, L. J. Reimert, 
OEMsr-740, Progress Report 16.5-44, New Jersey Zinc Co., 
Apr. 13, 1943. Div. 16-422.2-M2 

4. [Visibility of ultraviolet light sources and ranges of auto- 
collimating reflectors], OEMsr-740, Report 16.5-44, Uni- 
versity of Rochester, Vlarch 30, 1943. Div. 16-422.2-Ml 

5. A Special Radiation Filter Opaque to the Eye, Transmissive 

for Waves Shorter than 3,000 Angstroms, C. G. Abbot and 
L. B. Aldrich, OSRD 4169, OEMsr-1015, Final Report 

16.5-930, Smithsonian Institution. Div. 16-424-Ml 

6. Development of an Invisible Ultramolet Light Source, OEMsr- 

1073, Progress Report 16.5-78, University of California, 
Mar. 1, 1944. Div. 16-412-Ml 

7. Communication by Non-Visible Ultraviolet Radiation, 
Harvey E. White, OSRD 5378, OEMsr-1073, Service 
Projects NS-370 and NS-371, Final Report 16.5-112, Uni- 
versity of California, Oct. 15, 1945. Div. 16-450-Ml 


Chapter 7 

A UTOCOLLIMA TORS 

1. A System for Night Landing of Aircraft under Conditions of 
Blackout and Radio Silence, OSRD 741, OEMsr-69, Uni- 
versity of Rochester, July 24, 1942. Div. 16-433. 1-Ml 

2. A Demonstration Test of the Institute of Optics, Night Land- 

ing System at Wright Field, Supplementary Report to 
OSRD 741, OEMsr-69, University of Rochester, Aug. 10, 
1942. Div. 16-433. 1-M2 

3. A Visual Method for the Identification of Surface Vessels 
from Aircraft, Field Demonstration [at] Langley Field, Vir- 
ginia, and Norfolk, Virginia, September 19 to October 6, 
1942, OSRD 985, University of Rochester, Oct. 6, 1942. 

Div. 16-441-Ml 

4. A Field Demonstration Held for the Benefit of the Amphibious 

Forces in the Vicinity of Solomon’s Island, Virginia. Field 
Demonstration of Special Equip?nent for Night Landing 
Operations, OEMsr-427, Report 16.5-10, University of 
Rochester, Aug. 22 to 25, 1942. Div. 16-434. 1-Ml 

5. Methods of Making Triple Mirrors, Theodore Dunham, Jr., 

Walter S. Adams, and others, OSRD 3138, OEMsr-698, 
First Progress Report 16.5-52, Mount Wilson Observatory, 
February 1943. Div. 16-422. 3-Ml 

6. Methods of Making Triple Mirrors, Theodore Dunham, 

Jr., Walter S. Adams, and others, OSRD 3139, OEMsr- 
698, Second Progress Report 16.5-53, Mount Wilson Ob- 
servatory, Mar. 31, 1943. Div. 16-422. 3-Ml 

7. Air [to] Surface Vessel Identification System [Parts] II and 
III, OSRD 1598, OEMsr-725, Reports 16.5-13 and 16.5-14, 
University of Rochester, Mar. 1, 1943. Div. 16-441-M2-M3 

8. Aircraft Night Landing System, [Part] III, OSRD 1596, 

OEMsr-69, Interim Report 16.5-36, University of Roches- 
ter, June 30, 1943. Div. 16-433. 1-M4 


9. Aircraft Night Landing, [Part] IV, Visual Light Supple- 
ment, OEMsr-69, University of Rochester, Feb. 1, 1943. 

Div. 16-433. 1-M3 

10. Autocollimator Buttons, R. A. Woodson, OSRD 3370, 

OEMsr-994, Progress Report 16.5-64, covering period 
from July 1, 1943, to December 1, 1943, Eastman Kodak 
Co., Dec. 13, 1943. Div. 16-422.1-Ml 

11. Schmidt Autocollimator Unit, OSRD 3136, OEMsr-495, 

Report 16.5-60, Bausch and Lomb Optical Co., Dec. 10, 
1943. Div. 16-422-Ml 

12. The Manufacture of Autocollimating Lens Units, OSRD 

3948, OEMsr-932, Progress Report 16.5-87, Rochester 
Button Co., July 18, 1944. Div. 16-422. 1-M2 

13. Fluorescent Retrodirectional Autocollimating Buttons, cover- 

ing period [from] December 1 , 1943 to August 31, 1944 ^ 
Reginald T. Lamb and John H. McLeod, OSRD 4318, 
OEMsr-994, Final Report 16.5-96, Eastman Kodak Co., 
Sept. 27, 1944. Div. 16-422. 1-M3 

14. Schmidt Autocollimator Units, OSRD 5158, OEMsr-495, 

Service Project CE-11, Final Report 16.5-107, Bausch and 
Lomb Optical Co., June 30, 1945. Div. 16-422-M2 

15. Metaflectors, OSRD 6213, OEMsr-1000, Service Projects 

AC-225, NS-172, and others. Final Report 16.5-130, Uni- 
versity of Rochester, Oct. 4, 1945. Div. 16-422-M3 

16. Special Optical Devices, OSRD 5656, OEMsr-1219, Service 
Projects AC-225, NS-172, and others. Final Report 16.5- 
126, University of Rochester, Nov. 1, 1945. Div. 16-420-Ml 

17. Manufacture of Trihedral {Triple Mirror) Prisms, OSRD 
5652, OEMsr-698, Service Project AC-65, Final Report 

16.5- 122, Mount Wilson Observatory, Sept. 30, 1945. 

Div. 16-422.3-M3 

18. Triple Mirrors, W. V. Penfold, OEMsr-1319, Final Report 

16.5- 110, Penn Optical and Instrument Co., June 1, 1945. 

Div. 16-422.3-M2 

Chapter 8 

ANTIGLARE DEVICES 

1. Gradient Density Sun Glasses, OSRD 5177, OEMsr-989, 
Service Project QMC-37, Final Report 16.5-109, Bausch 
& Lomb Optical Co., Nov. 15, 1945. Div. 16-423-M4 

2. The Development of a Sight in Which the Solar Glare Is 
Eliminated, Donald H. Menzel, OEMsr-571, Progress Re- 
port 16.5-39, Harvard University, Dec. 31, 1942. 

Div. 16-423-Ml 

3. An Investigation of Sun-Occulting Devices, James R. Balsley 
and Harlow Shapley, OSRD 4097, OEMsr-571, Final Re- 
port 16.5-90, Harvard University, July 3, 1944. 

Div. 16-423-M3 

4. Telescope Shutter, Sun-Obscuring Device, C. M. Tuttle, 
OSRD 3404, OEMsr-996, Problem DD-2528, Progress Re- 
port 16.5-75, Eastman Kodak Co., Feb. 14, 1944. 

Div. 16-423-M2 

5. Special Optical Devices, OSRD 5656, OEMsr-1219, Service 

Projects AC-225, NS-172, and others. Final Report 16.5- 
126, University of Rochester, Nov. 1, 1945, Volume 4, 
Development of the Icaroscope. Div. 16-420-Ml 


Chapter 9 

MISCELLANEOUS OPTICAL DEVELOPMENTS 

1. Air [to] Surface Vessel Identification System [Part] II, 
OSRD 1598, OEMsr-725, Report 16.5-13, University of 
Rochester, Mar. 1, 1943. Div. 16-441-M2 


140 


BIBLIOGRAPHY 


2. Air [to] Surface Vessel Identification System [Part] III, 

Special Equipment Developed for Identification of Surface 
Vessels from the Air, OEMsr-725, Report 16.5-14, Uni- 
versity of Rochester, Mar. 1, 1943. Div. 16-441-M3 

3. Development of Kellner-Schmidt Optical Systems [Part] I, 

OEMsr-510, Interim Report 16.5-16, University of Roch- 
ester, June 30, 1943. Div. 16-421. 21-Ml 

4. Development of Kellner-Schmidt Optical Systems [Part] II, 

OEMsr-510, Interim Report 16.5-17, University of Roches- 
ter, Aug. 15, 1943. Div. 16-421. 21-Ml 

5. Wide-Aperture Kellner-Schmidt Optical Systems, OEMsr- 

510, Progress Report 16.5-24, University of Rochester, 
Aug. 15, 1943. Div. 16-421. 21-M2 


6. Air [to] Surface Vessel Identification System [Part] IV, 
The Identification of Surface Vessels from Aircraft Equipped 
with Linked Searchlight and Antioscillation Mounted Bin- 
oculars, OSRD 3128, OEMsr-725, Report 16.5-59, Uni- 
versity of Rochester, Oct. 15, 1943. Div. 16-441-M4 

7. Special Optical Devices, OSRD 5656, OEMsr-1219, Service 
Projects AC-225, NS-172, and others. Final Report 16.5- 
126, University of Rochester, Nov. 1, 1945. 

Div. 16-420-Ml 

7a. Ibid., Vol. 1, Kellner-Schmidt Strip Projector. 

7b. Ibid., Vol. 2, Cadillac-2. 

7c. Ibid., Vol. 3, Flash Metascope. 


R 


OSRD APPOINTEES 

DIVISION 16 


Chief 

George R. Harrison 


Deputy Chiefs 


Paul E. Klopsteg 

Consultants 

Richard C. Lord 

Herbert E. Ives 

Technical Aides 

F. E. Tuttle 

H. R. Clark 


Richard C. Lord 

J. S. Coleman 

Members 

H. K. Stephenson 

0. S. Duffendack 


Arthur C. Hardy 

Theodore Dunham, Jr. 


Herbert E. Ives 

E. A. Eckhardt 


Paul E. Klopsteg 

Harvey Fletcher 


Brian O’Brien 

W. E. Forsythe 

SECTION 16.1 

Chief 

F. E. Tuttle 


Theodore Dunham, Jr. 


G. W. Morey 
F. L. Jones 


Lillian Elveback 


Ira S. Bowen 
W. V. Houston 


Consultants 

H. F. Mark 
H. F. Weaver 

Technical Aides 

S. W. McCtJSKEY 

H. F. Weaver 
Members 

R. R. McMath 
G. W. Morey 

F. E. Wright 


SECTION 16.2 

Chief 

Brian O’Brien 

Consultants 

W. R. Brode V. K. Zworykin 

Technical Aide 
Chas. E .Waring 


Members 

Julian H. Webb 


W. E. Forsythe 


Harvey E. White 


OSRD APPOINTEES {Continued) 



SECTION 16.3 

Lewis Knudson 

Chief 

Arthur C. Hardy 

Consultants 

Parry H. Moon 
Edward R. Schwarz 

S. Q. Duntley 

Technical Aides 

Arthur W. Kenney 

Ernest T. Larson 

Edwin G. Boring 

Members 

L. A. Jones 

F. C. Whitmore 

W. L. Enfield 

SECTION 16.4 

Chief 

0. S. Duffendack 

Consultants 

H. G. Houghton, Jr 

W. H. Radford 

H. S. Bull 

Technical Aides 

Winston L. Hole 

James S. Owens 

Alan C. Bemis 

Saul Dushman 

Members 

H. G. Houghton, Jr 
George A. Morton 

SECTION 16.5 

W. E. Forsythe 

Chiefs 

Herbert E. Ives 

W. E. Forsythe 

Deputy Chiefs 

Brian O’Brien 

E. Q. Adams 

Consultants 

A. C. Downes 

Technical Aides 


William Herriott John T. Remey 

Val E. Sauerwein 


D. W. Bronk 

A. C. Hardy 
Theodore Matson 

A. H. Pfund 

Members 

W. B. Rayton 

A. B. Simmons 

G. F. A. Stutz 
Harvey E. White 


V. K. Zworykin 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS 


Contract Number 

Name and Address of Contractor 

Subject 

NDCrc-19 

Precision Castings Company, Inc. 

Syracuse, New York 

Producing metallic mirrors by a metal-spraying 
process. 

NDCrc-202 

University of Rochester 

Rochester, New York 

Apparatus for improvement of night vision. 

OEMsr-69 

University of Rochester 

Rochester, New York 

Night landing of aircraft. 

OEMsr-81 

University of Rochester 

Rochester, New York 

Development of infrared phosphors. 

OEMsr-115 

Carnegie Institution of Washington 
Washington, D. C. 

Investigations in connection with night sky- 
scanning. 

OEMsr-169 

Radio Corporation of America 

RCA Victor Division 

Camden, New Jersey 

Construction of 2 infrared viewing tubes — infrared 
(light sources: 1 -f TRA-115 gasoline driven 
generator). 

OEIMsr-188 

The Johns Hopkins University 

Baltimore, Maryland 

Infrared penetration of gases and vapors. 

OEMsr-249 

Precision Castings Company, Inc. 

Syracuse, New York 

Search-light mirrors using sprayed metal backs. 

OEMsr-265 

University of Rochester 

Rochester, New York 

Brightness of night sky. 

OEMsr-303 

University of Rochester 

Rochester, New York 

Development of an aspheric molding machine for 
luminescent traffic markers. 

OEiMsr-427 

University of Rochester 

Rochester, New York 

Investigation of night markers for harbors and 
beaches. 

OEMsr-440 

Radio Corporation of America 

RCA Victor Division 

Camden, New Jersey 

Special purpose image tubes and infrared phosphors. 

OEMsr-472 

University of Rochester 

Rochester, New York 

Field lens for use in connection with the Navy- 
RCA “Block Project 135.” 

OEMsr-623 

Eastman Kodak Company 

Rochester, New York 

Triplet projection lens in plastics. 

OEMsr-698 

Carnegie Institution of Washington 

Washington, D. C. 

Development of triple mirrors. 

OEMsr-740 

The New Jersey Zinc Company (of Pa.) 

New York, New York 

Luminescent markers and IR phosphors. 

OEMsr-766 

Western Electric Company, Incorporated 

New York, New York 

Night surveying and signaling by infrared. 

OEMsr-951 

Radio Corporation of America 

RCA Victor Division 

Camden, New Jersey 

Special purpose infrared viewing equipment. 

OEMsr-982 

Polytechnic Institute of Brooklyn 

Brooklyn, New York 

Development of phosphors and basic phosphor 
material. 

OEMsr-987 

Ohio State University Research Foundation 
Columbus, Ohio 

Infrared transmitting filter development. 

OEMsr-996 

Eastman Kodak Company 

Rochester, New York 

Sun scanning devices. 

OEMsr-1000 

University of Rochester 

Rochester, New York 

Infrared auto-collimators. 


143 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS {Continued) 


Contract Number 

Name and Address of Contractor 

Subject 

OEMsr-1031 

Radio Corporation of America 

RCA Victor Division 

Camden, New Jersey 

Crystal light source. 

OEMsr-1041 

Eastman Kodak Company 

Rochester, New York 

S])rayed metal mirrors. 

OEMsr-1073 

The Regents of the University of California 
Berkeley, California 

Sources of ultraviolet radiation. 

OEIMsr-1075 

The Trustees of the University of Pennsylvania 
Philadelphia, Pennsylvania 

Mounts for infrared driving telescopes. 

OEMsr-1100 

Eastman Kodak Company 

Rochester, New York 

Metascope design. 

OEMsr-1155 

General Electric Company 

Schenectady, New York 

Infrared phosphors. 

OEMsr-1219 

University of Rochester 

Rochester, New York 

Special optical devices. 

OEMsr-1319 

Penn Optical & Instrument Company 

Pasadena, California 

Fabrication of high precision triple mirrors. 


144 


SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive Secretary, 
National Defense Research Committee [NDRC], from the War or 
Navy Department through either the War Department Liaison 
Officer for NDRC or the Office of Research and Inventions (formerly 
the Coordinator of Research and Development), Navy Department. 


Service 

Project 

Number Subject 


AC-15 
AC-15, Ext. 


AC-37 

AC-64 

AC-65 

AC-66 

AC-103 

AC-104 

CE-7 

CE-11 

CE-11, Ext. 

CE-11, Ext. 

CE-34 

CE-34, Ext. 

CWS-8 

N-103 

NA-175 

NR-108 

NS-158 

NS-172 

NS-279 

NS-282 

NS-350 

NS-370 

QMC-37 

SOS-5 

OD-16 

OD-150 

SC-21 

SC-122 


Night landing ultra-violet light with minimum enemy observation. 

Development of light source producing ultra-violet light of sufficient intensity for illuminating instrument 
boards. 

Development of an aircraft window suitable for photographic purposes. 

Phosphor light sources. 

Triple mirrors. 

Night identification. 

Identihcation for B-29 aircraft. 

Infrared radiation from exhaust systems. 

Mirrors of stainless steel or metal other than copper. 

Luminous material to replace radio-active substances. 

Ultra-violet reflector buttons. 

Development of a phosphorescent material. 

Image forming IR equipment. 

Infrared filters. 

Generation of colored smokes. 

Development of air-surface identification equipment. 

Minimum visibility aids to aircraft carrier landings. 

Night vision adaptometer — phosphorescence warning and quenching. 

Development of range finding attachment for infrared radiation receivers. 

Infrared phosphors. 

Radioactive luminous markers for shipboard use. 

Determination of time response characteristics of electron image tubes and phosphor type image forming. 
Development of a large aperture Kellner-Schmidt system for the large type of infrared receiver, etc. 
Development and improvement of invisible ultra-violet sources. 

Graded sun glasses. 

Means for locating bunkers. 

t 

Blackout lighting. 

Apparatus for measuring the integrated intensity and the time variation of the intensity of muzzle flash. 
Photography of anti-aircraft shell bursts. 

Infrared coating of lenses. 





145 


INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 
For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Activators 

dominant and auxiliary phosphors, 47 
infrared sensitive phosphors, 55-57 
Aerial photography at night with strip 
projectors, 128-130 

Afterglow in phosphors, definition, 54 
Airplane identification with infrared, 31 
Airplane landing triple mirror device, 78 
Airplane operations infrared instru- 
ments 

B-29 tailsight telescope, 31 
paratroop assembly systems, 32 
telescope for night landing, 31-32 
Airplane runway markers 

high-pressure mercury arc sources, 
93-94 

metaflector, 112, 113 
triple mirror autocollimators, 2, 118 
ultraviolet autocollimators, 112-113 
Alkali metals as photoemitters, 8 
Alpha particle excitation 
exhaustion of phosphor, 48 
phosphors, 41-42 

scintillations produced by particles, 
49 

American Optical Company, greenblock 
manufacture, 49 

Anode lenses for image tubes, 16 
Antiglare devices, 120-126 
for aircraft detection, 121-126 
graded-density goggles, 120-121 
icaroscope method, 124-126 
occulting disk method, 121-122 
shutter method, 122-124 
Arc lamps, 82-90 

carbon-arc lamps, 82-84 
mercury-vapor lamps, 84-90 
Aspheric corrector plates, dropping 
process, 49-51 

greenblock, refractory material, 49 
mold surface curves, 51 
plate testing, 50-51 
production, 49 
size of plates, 51 
suction molding, 49-50 
Aspheric corrector plates, greenblock 
cutting, 51-53 

1 to 1 contour machine, 51-53 
5 to 1 contour machine, 53 
Aspheric corrector plates, metasocpes, 
49-53 

construction, 49-53 
dropping process of production, 49-51 
greenblock curves, cutting, 51-53 
Astigmatism with image tube, 6 
Autocollimators, 110-119 
construction, 105 
gallium lamp use, 98 
general description, 2 
Kellner-Schmidt autocollimators, 
110-114 

metaflectors, 113-114 


principles of operation, 110 
range, 98 

triple mirror autocollimators, 114- 
119 

ultraviolet autocollimators, 110-113 

B-1 phosphor 

preparation, 62-63 
time-lag and inertia measurement, 37 
selenide content, 59-60 
B-17 airplane, CIC unit in bomb bay, 
130-131 
B-29 airplane 

sweep mount for metascope, 128 
tailsight telescope, 31 
triple-mirror autocollimator, 119 
Background of phosphors (afterglow), 
54 

Bausch and Lomb Optical Company 
autocollimator molds, 110 
graded-density goggles, 120-121 
Binoculars 

night aerial identification of vessels, 
132-133 

night-driving instruments, 28 
periscopic, 28 
Brightness 

incandescent filament lamp, 76 
mercury-vapor lamps, 89-90 
target brightness at night with glare 
elimination, 133 
British infrared beacon, 78 
Brooklyn Polytechnic Institute, phos- 
phor powders for type A meta- 
scope, 38 

Buttons in phosphors 
definition, 55 
early production, 56-57 
formation, with Standard VI phos- 
phor, 62-63 

Butvar as a magnesium spark filter 
component, 108-109 

Cadillac-2, CIC screen and projector, 
130-132 

B-17 installation, 130-131 
projector specifications, 132 
screen specifications, 131-132 
Carbon-arc, high intensity ultraviolet 
source, 100-102 

current-modulated flame arc, 102 
grid-modulated carbon arc, 102 
searchlight, 24 inch, 100-102 
triple-prism return, 101-102 
Carbon-arc lamps, 82-85 
electrical characteristics, 84 
manufacturers, 83 
military applications, 83 
radiation characteristics, 84, 85 
Cerium as a phosphor activator, 58 
Cesium surfaced photocathodes, prepa- 
ration, 8 


Chromatic aberrations in image tube, 6 
CIC installed in B-17 airplane, 130-131 
Color temperature, incandescent fila- 
ment lamps, 76 

Continental Lithograph Corporation, 
phosphor for ultraviolet auto- 
collimators, 111 

Contour machines for greenblock cut- 
ting, 51-53 

Contrast effect, image tubes, 12-14 
Conversion, infrared, 11-12 
Corex 9863 filter, 100-101 
Corning filters for metascopes 
No. 428, light blue green, 37 
No. 2450; 39 

anklo infrared absorbing, 37 
removable, 39 

Corona in power supply units, 22 
Current-modulated flame arc, 102 
Curvature of image field lens, 5-6, 
10-11 

Decay phenomena, phosphors, 68-69 

Eastman Kodak Company 

autocollimator manufacture, 110 
graded-density goggles, 120 
Kxx film for K-S strip projector, 130 
type A metascope production, 38 
type B metascope production, 39 
Electron optical considerations, infra- 
red image tube, 5-7 
Electronics Laboratories of Indianapo- 
lis, metascope production, 43 
Electrostatic lens, 5 
Emission spectrum 
gallium lamp, 96 
phosphors, 47 

Evaporated film goggles, 120-121 
Excitation of phosphors 
after partial exhaustion, 68 
definition, 54 
measurement, 63 
methods of excitation, 48-49 
within instrument, 37 
Exhaustion of phosphors 
curves, 66-67 
definition, 55 

Extinction in phosphors, definition, 
54-55 

Field emission, cold discharge of infra- 
red telescope, 13 
Filament lamps, incandescent 
see Incandescent filament lamps 
Filament sources, incandescent lamps, 
78 

Film evaporation on glass surface, 120- 
121 

Filters 

anti-sun glare, for aircraft detection, 
121 


INDEX 


147 


Corex 9863; 100-101, 106 
goggle filters for high pressure iiier- 
ciiry arcs, 109 
magnesium spark, 108 
mercury-vapor lamps, 90 
standard VI phosphor, 57-58 
ultraviolet autocollimators. 111 
ultraviolet sources and receivers, 
106-109 

ultraviolet with type L metascope, 45 
Wratten No. 88 for strip projector, 
130 

Filters, metascope, 37-40 
Corning No. 428; 37 
Corning No. 2540; 39 
Corning anklo infrared absorbing, 37 
Corning removable, 39 
excitation, 48-49 
infrared, 40 
red for type A, 38 
XRX7; 38 

Filters, ultraviolet sources and receiv- 
ers, 106-109 

G-filter for gallium lamp, 106-108 
goggle filter for mercury arc, 109 
magnesium spark, 108 
nickel, 106 

photomultiplier-tube receiver, 108- 
109 

security measures, 106 
Flash metascope, 127-128 
phosphors used, 127 
sweep mechanism, 128 
Fluorescence, ultraviolet, 93 
Fluorescent screen phosphors, 9 
Fluxes used with phosphors, 55, 62 
Focal surface curvature in image tubes, 
5-6, 10-11 
Formulas 

chromatic aberration, 6 
decay phenomena, phosi^hors, 68-69 
image tube sensitivity, 12 
resolution of image tubes, 10 
telescope sensitivity, 22, 23 
ultraviolet range, 91-93 
FP54 electrometer tube amplifiers, 
phosphor characteristics meas- 
urement, 64 

Fused-quartz triple mirrors, 105 

Gallium lamp, 94-98 
Autocollimator use, 98 
construction, 95-97 
development, 94 
discharge tube, 95-97 
emission spectrum, 96 
portable unit, 97-98 
purification of gallium, 96 
range, 94 
signalling use, 97 
visibility, 97 
voice transmission, 98 
Geiger-counter receiver, 105-106 
General Electric Company 
filament lamps, 70 
gallium lamp testing, 94 
infrared-sensitive phosphor develop- 
ment, 54-69 


infrared telescope, 4 
German listening gear, 229 
Germicidal lamp, 86, 90 
G-filter for gallium lamp, 107-108 
Glare elimination, night aerial identifi- 
cation of vessels, 133 
Glide landings, triple-mirror autocolli- 
mators, 118-119 

Glider telescopes for towing and land- 
ing, 30, 31 

Goggle-filter for a mercury arc, 109 
Goggles, graded density, 120-121 
design, 120 

evaporated film, 120-121 
Grid- modulated carbon arc, 102 

Harvard University 
antiglare devices, 122 
graded-density goggles, 120 
Hayward Optical Glass Company, 
triple mirror production, 116 
High pressure mercury arcs, 93-94 

Icaroscope, antiglare device, 124-126 
formation of screens, 126 
general description, 3 
phosphor requirements, 126 
phosphorescent saturation, 124 
type 1 icaroscope, sun telescope, 
124-125 

type 2 icaroscope, lightweight, 125 
Image field lens, curvature, 5-6, 10-11 
Image tube development, 33, 34 
Image tubes, infrared 
see Infrared image tubes 
Image-forming infrared receivers 
see Metascopes 

Incandescent filament lamps, 71-81 
applications, 71-74 
brightness, 76 

design and construction, 70-75 
electrical characteristics, 71-76 
lamp life, 71-74 
luminous efficiency, 76 
miscellaneous filament sources, 78 
modulating equipment, 75 
operating characteristics, 75-76 
power supply, 71-76 
radiation characteristics, 76-77 
recommended lamps for military 
projects, 78-81 
spatial distribution, 77 
spectral distribution, 71-76 
tungsten lamps, 70-78 
Inertia in phosphors, definition, 55 
Infrared autocollimators 

see Metaflectors, infrared autocolli- 
mators 

Infrared beacon, 78 
Infrared image tubes, 4-22 
basic components, 4 
electron optical consideration, 5-7 
fluorescent screen phosphors, 9 
1P25; 4-14, 24 
photocathodes, 7-9 
production, 4 

research recommendations, 17, 34 
standardized type, 5 


Infrared image tubes, experimental 
types, 14-17 

high voltage tubes, 15-16 
low-magnification tubes, 15-16 
single voltage tubes, 14-16 
Infrared image tubes, performance, 
10-14 

contrast, 12-14 
conversion, 11-12 
resolution, 10-11 
sensitivity, 11-12 

Infrared image tubes, power supplies, 
17-22 

converter unit, 17, 18 
power units, 19-22 
voltage divider, 18, 19 
Infrared receivers 
see Metascopes 
Infrared sources, 70-90 
applications, 70 
arc lamps, 82-90 

incandescent filament lamps, 70-81 
Infrared telescope, 22-34 
basic components, 4 
construction, 33-34 
production, 5 
use, 1, 33-34 

Infrared telescope performance, 22-24 
expected range, 23-24 
resolving power, 23 
sensitivity, 22-23 
Infrared telescope types, 23-33 
airborne operations instruments, 30- 
32 

night-driving instruments, 28-30 
reconnaissance type, 26-28 
signaling and marker detectors, 23-26 
special types, 32-33 
Institute of Optics, University of 
Rochester 

infrared phosphors, 47 
K-S projector for infrared visibility, 
78 

metascope development, 35 
ultraviolet radiation, 91 

Jeep movement at night, infrared 
telescope, 1 

Kellner-Schmidt 

corrector plates produced by drop- 
ping process, 51 

metaflectors, infrared autocollima- 
tors, 113-114 
oi)tic system, 35-36 
solid optic system, 44 
ultraviolet autocollimators, 110-113 
Kellner-Schmidt strip projector, 78, 
128-130 
design, 129 
development, 129 

low-altitude aerial photography at 
night, 128 
optical system, 129 
performance, 129-130 
use, 129-130 



148 


INDEX 


Kodak film for strip projector, 129-130 
KR31 rectifier, 18, 20 

Lamp life, incandescent filament lami)s, 
71-74 

Leakage in image tubes, 14 
Leakage in power supply units, 22 
Lenard, research on sulfide phosphors, 
62 

Lens structure of image tubes, 14 
Light-sun, energy emitted by a phos- 
phor, 54 

Low-altitude aerial photogi'aphy at 
night, strip projector, 128-130 
Low-pressure mercury arcs, 94 
Lucite, metascope cases, 44 
Luminescence in phosphors, definition, 
54 

Luminous efficiency, incandescent fila- 
ment lamps, 76 

Magnesium spark, 98-100 
description, 98 
electrical circuits, 98-99 
filter, 108 

hand held magnesium source, 100 
large magnesium source, 99 
radar principles, 100 
spark intensities, 99 
Magnetic lens, 5 
Magnification in image tube, 5 
Mark VIII gunsight, 31 
Marker and signaling detectors, 23-26 
early telescopes, 24, 25 
portable telescopes, 26 
Mercury arcs, high pressure, 93-94 
Mercury arcs, low pressure, 94 
Mercury- vapor lamps, 84-90 
air-blast cooled, 89 
applications, 85 
design and construction, 86 
electrical characteristics, 87 
general characteristics, 84 
source lamps, 85 

Mercury-vapor lamps, operating char- 
acteristics 
lamp life, 87 

modulating equipment, 87 
Mercury-vapor lamps, radiation, 87-90 
beam output, 90 
brightness, 89-90 
filters, 90 

spectral distribution, 87-90 
Metaflectors, infrared autocollimators, 
113-114 

43 ^-inch metaflector, 112-113 
4^-inch metaflector, 113-114 
8-inch metaflector, 114 
airplane runway markers, 112 
charging requirements, 113 
featherweight unit, 113 
small boat landing, 112-113 
Metascopes, 35-53 
aspheric corrector plates, 49-53 
basic construction, 35-36 
basic principle, 35 

flash metascope, sensitive to visible 
radiation, not infrared, 127-128 


optical design, 35, 36 
phosphor application, 47-49 
pressure-sealing, 42 
stadiameter attachments, 46 
ultraviolet, 104 
use, 2 

Metascopes, types, 38-46 

A, 36-38 

Al, pressure-sealed, 41 
A-M, modified type A, 40, 41 

B, 38-39 

F, radium excited, 41-43 
H, 43, 44 

J, single hand operation, 43 

K, solid system, 44 

L, 44-46 

M and O miniature, 39, 40 
Mold surface curves in aspheric cor- 
rector plates, 51-53 
Mount Wilson Observatory 

fused-quartz triple mirrors, 105 
fused-quartz triple prisms, 101-102 
triple mirror autocollimators, 110 
triple mirror glass, 114 
triple mirror production, 116 

National Carbon Company 
carbon for arc-lamps, 82-83 
ultraviolet carbon, 100 
Navy searchlight, infrared, 82 
New Jersey Zinc Company 

infrared-sensitive phosphor develop- 
ment, 54-69 
ultraviolet radiation, 91 
Nickel filter, semisolid, 106 
Nickel-chromium film on glass surface, 
120-121 

Night aerial identification of vessels, 
132-133 

apparatus specifications, 133 
binoculars, 133 
glare elimination, 133 
searchlights, 133 

Night aerial photography at low alti- 
tudes with strip-projectors, 128- 
130 

Night aerial reconnaissance with flash 
metascopes, 127 

Night air-sea rescue with triple-mirror 
autocollimators, 119 
Night torpedo bombing training with 
triple-mirror autocollimators, 
119 

Night-driving instruments, 28-30 
binoculars, 28 
helmet instruments, 28 
incandescent filament lam]}s, 78-79 
periscopic binoculars, 28 
protectoscope, 28 
Night-landing aircraft 
recommended lamps, 80 
telescopes, 31, 32 

triple-mirror autocollimators, 117- 
119 

ultraviolet autocollimators, 112-113 
Norway-vapor lamps, operating char- 
acteristics, 85-87 

Occulting-disk, antiglare device, 121- 


122 

principle, 121-122 

type 1 instrument, disk inside tele- 
scope, 122 

type 2 instrument, disk outside tele- 
scope, 122 
1P25 image tube 
construction, 10 
electrode design, 6-7, 13 
infrared telescope use, 24 
manufacture and use, 4 
multiple voltage, 14 
phosphor materials, 9 
Optical devices sensitive to visible light, 
127-133 

Cadillac-2, CIC screen and projector, 
131 

flash metascope, 127-128 
for night aerial identification of 
vessels, 132-133 
K-S strip projector, 128-130 

P-70 airplane, night landing, 118 
Paratroop equipment 
infrared telescope, 2, 32 
triple-mirror autocollimators, 118- 
119 

Penn Optical Company, triple-mirror 
production, 116 
Phosphor 

activators, 47, 56-57 
composition, 54 

definitions of terminology, 54-55 
fluorescent screen use, 9 
icaroscope antiglare device, 124-126 
metascope use, 47-49 
ultraviolet autocollimator use. 111 
Phosphor characteristics, measurement, 
63-56 

excitation, 63 
resolving power, 65-66 
sensitivity, 66 

spectral measurements, 64-65 
stimulability, 63-64 
threshold, 64 
time-lag, 66 
Phosphor pencil, 131 
Phosphor preparation, 61-63 

fluxes, influence on preparation, 62 
purity of materials, 61-62 
standard phosphors, 62-63 
Phosphor properties 

alpha-particle excitation, 41-42 
definitions, 54-55 
emission spectra, 47 
excitation, 37, 48-49 
exhaustion, 42, 54 
maximum sensitivity, 42 
optical properties of standards, 47-48 
})hysical properties of standards, 48 
stimulation spectra, 47 
Phosphor terminology, 54-55 
Phosphor theory, 66-69 
decay phenomena, 68-69 
excitation after partial exhaustion, 68 
exhaustion curves, 66-67 
saturation, 68 

thermal and stimulated emission, 68 


I 


--i'Af 


INDEX 


149 


Phosphor types, 55-61 

standards I through V, early phos- 
phors, 55 

selenide phosphors, 59-60 
standard VI, 55-58 
standard VII, 58-59 
zinc-sulfide phosphors, 60-61 
Phosphorescent saturation principle, 
124 

Photocathodes, 6-9 
cesium surface, 8 
spectral sensitivity, 9 
thermionic emission, 8 
Photoelectric receivers, 104-105 
Photoglow tubes, 105-106 
Photographic film, Eastman Kxx for 
K-S strip projector, 129-130 
Photomultiplier-tube, receiver filter, 
108-109 

Pittsburgh Plate Glass Co., 1045X 
glass, 49 

Plane-to-plane identification with triple- 
mirror autocollimators, 119 
Plexiglas, Cadillac-2 screen, 131 
Point-source sensitivity with infrared 
telescope, 22 
Polaroid Corporation 
metascope filters, 38 
metascopes, plastic construction, 45 
Polytechnic Institute of Brooklyn, in- 
frared-sensitive phosphor devel- 
opment, 54-69 

Pressure-sealing, metascopes, 42 
Prisms, fused-quartz triple, 105 
Protectoscope (night tank driving in- 
strument), 28 

Quantum efficiency of phosphors, 48, 55 
Quenching phosphors, 54 

Radar principles, magnesium spark 
applications, 100 
Radio Corporation of America 

infrared-sensitive phosphor develop- 
ment, 54-69 
Princeton laboratories, 4 
Radium excited metascope, 41-43 
Range of the ultraviolet, 91-93 
RCA 1P28, gallium lamp receiver, 98 
RCA 1654, thermionic rectifier, 18-20 
RCA tube IP28, photoelectric receiver 
construction, 104 

Receivers, image-forming infrared 
see Metascopes 
Receivers, ultraviolet 
see Ultraviolet receivers 
Reconnaissance telescopes, 26-28 
early, experimental, 26-27 
snooperscope and sniperscope, 28 
Rectifier in image tube power supplies, 
17, 18 

Regeneration of phosphors, 48 
Research recommendations for infrared 
image tubes, 17, 34 
Resolving power 
image tube, 10-11 
infrared telescopes, 23 
phosphors, 65-66 


Rochester Button Company, autocolli- 
mator development, 110 
Rohm and Haas Company, Philadel- 
phia, plexiglas manufacture, 132 

Samson United Corporation, metascope 
production, 38, 41, 43 
Saturation, phosphors, 68 
Searchlights 

Navy 24-inch searchlight, 100-101 
night aerial identification of vessels, 
132-133 

source of infrared radiation, 1 
Selenide phosphors, 59-60 
Sensitivity 

infrared image tube, 11-12 
phosphor measurement methods, 66 
rating in metascopes, 38 
Shutter antiglare device, 122-124 
photoelectric process, 122 
photographic process, 123-124 
Signaling and marker detectors, 23-26 
early experimental telescopes, 24-25 
portable telescopes, 26 
Single band operation metascope, 43 
Sniperscope 

construction, 28 

filament lamp-recommendation, 81 
reconnaissance use, 28 
Snooperscope 

filament lamp-recommendation, 81 
general description, 2, 28 
paratroop use, 32 
reconnaissance use, 28 
Solid system metascope, 44 
Sonne camera, S-6 and S-7, 128-130 
Spatial distribution, filament lamps, 77 
Spectral distribution 

filament lamps, 71-74, 76-78 
mercury-vapor lamps, 87-90 
Spectral sensitivity, photocathodes, 9 
Spontaneous emission in phosphors, 
definition, 54 

Stadiameter attachments, meta- 
scopes, 46 

Stimulability of phosphors 
definition, 55 
factors effecting, 47 
measurement, 63 
Stimulation of phosphors 
definition, 54 
spectra, 47 

Strip camera projectors, 128-130 
Suction molding, aspheric corrector 
plate, 49-50 

Surface vessel identification with ultra- 
violet autocollimators, 112-113 
Sweep mount for metascope, 128 

Tank movements at night 
infrared telescope, 1 
protectoscope, night driving instru- 
ment, 28 

TBD-1 airplane, autocollimator testing, 
118 

Telescopes 

glider operations, 30 
reconnaissance, 26-28 


tailsight telescope for B-29; 31 
ultraviolet, 36-38 
Telescopes, infrared 
see Infrared telescope 
Television, image tube in pick-up de- 
vice, 5 

Thermionic emission 
infrared telescope, 13 
phosphors, 68 
photocathodes, 8 

Thermionic rectifier, RCA 1654; 18-20 
Threshold sensitivity 
phosphors, 48, 64 
standard VI phosphor, 57-58 
Time lag, phsophors 
definition, 55 

methods of measurement, 66 
Triple mirror autocollimators, applica- 
tions, 117-119 
glider landings, 118-119 
night air-sea rescue, 119 
night landing, carrier-based planes, 
118, 119 

night landing, enemy territory, 117- 
118 

night landing, ground based planes, 
118 

night torpedo bombing training, 119 
plane-to-plane identification, 119 
Triple mirror autocollimators, manu- 
facturing process. 116-117 
milling, 116-117 
mounting, 117 
production results, 116 
quality verification, 117 
Triple mirror autocollimators, proper- 
ties, 114-115 
Triple mirror devices 
autocollimators, 114-119 
fused quartz, 105 
general description, 2 
Triple-prism return, carbon arc source, 
101-102 
Tubes, image 
see Image tubes 

Tungsten lamps, incandescent filament, 
70-78 

24-inch searchlight, 100-102 

Ultraviolet autocollimators, 105-106, 
110-113 

construction, 110 
filters, 111-112 
phosphors. 111 
photoglow tubes, 105-106 
range, 110-111 

triple mirrors of fused quartz, 105 
Ultraviolet autocollimators, applica- 
tions, 112-113 

night landing of aircraft, 112-113 
sea-search, 112-113 
small boat landings, 112-113 
Ultraviolet carbon, 100 
Ultraviolet radiation, 91-93 
fluorescence, 93 
range, 91-93, 108-109 
regions, 91 


150 


INDEX 


Ultraviolet receivers, 104-106 
Geiger-counter receiver, 105-106 
metascopes, 104 

photoelectric receivers, 104-105 
Ultraviolet sources, 93-102 

carbon-arc, high intensity, 100-102 
filters, 106-109 
gallium lamp, 94-98 
magnesium spark, 98-100 
mercury arcs, high pressure, 93-94 
mercury arcs, low pressure, 94 
Ultraviolet spectrum, visibility to naked 
eye, 111-112 

Ultraviolet telescope, metascope fore- 
runner, 36-38 

United States Radium Company, flash 
metascope phosphor, 127 
University of California 

autocollimator development, 105 
ultraviolet radiation, 91 


University of Pennsylvania, Johnson 
Foundation, 4 
University of Rochester 
autocollimator research, 105, 110-111 
glass mold production, 45 
graded-density goggles, 120 
icaroscope development, 124 
infrared-sensitive phosphor develop- 
ment, 54-69 

triple mirror research, 114 
ultraviolet autocollimators, 112 

Vance amplifier, phosphor character- 
istic measurement, 65 
Vibrator in image tube power supplies, 
17, 18 

Visible light, optical developments 
see Optical devices sensitive to visible 
light 




lUTryTUTflfWiD 


Voice transmission of gallium lamp, 
98 

Voltage focusing, infrared image tube, 
11 

Water-cooled capillary lamp, 88 

Willemite screen 
conversion, 12 
fluorescent screens, 9 
light output, 15 

XRX7 filter, metascope use, 38 

Zeiss graded density goggles, 120 

Zinc-sulfide phosphors, 60-61 
bases, 61 
emission, 61 
operations, 61 
stimulation, 61 


I 
















